Core APIs, Part 2 295

keyboardDidShow = () => {

this.setState({

keyboardWillShow: false,

keyboardVisible: true,

});

// ...

};

keyboardWillHide = (event) => {

this.setState({ keyboardWillHide: true });

// ...

};

keyboardDidHide = () => {

this.setState({

keyboardWillHide: false,

keyboardVisible: false

});

};

// ...

The listeners keyboardWillShow, keyboardDidShow, and keyboardWillHide will each be called with

an event object, which we can use to measure the contentHeight and keyboardHeight. Let’s

do that now, using this.measure(event) as a placeholder for the function which will perform

measurements.

messaging/components/KeyboardState.js

// ...

keyboardWillShow = (event) => {

this.setState({ keyboardWillShow: true });

this.measure(event);

};

keyboardDidShow = (event) => {

this.setState({

keyboardWillShow: false,

keyboardVisible: true,

});

this.measure(event);

Core APIs, Part 2 296

};

keyboardWillHide = (event) => {

this.setState({ keyboardWillHide: true });

this.measure(event);

};

// ...

For iOS it would be sufficient to calculate measurements in the keyboardWill* events, since the

keyboardDid* events should receive the same event parameter. However, since Android only

supports the keyboardDid* events, we also need to use keyboardDidShow. Calculating measurements

in keyboardDidShow on iOS shouldn’t affect the app’s behavior, but we could do this conditionally

by checking Platform.OS === 'android' if we preferred.

We can use these events to keep track of the keyboard’s current state. Each event object will have

the following properties:

• duration - Duration of the keyboard animation. In practice, this is typically constant across all

keyboard animations. This property only exists on iOS, so we’ll use a constant to approximate

it on Android.

• easing - Easing curve used by the keyboard animation. This will be the special easing curve

called 'keyboard', which we can use to sync our own animations with the keyboard’s. This

property only exists on iOS, since there isn’t a specific keyboard animation on Android. We’ll

use 'easeInEaseOut' as a pleasant-looking default to approximate the keyboard animation on

Android.

• startCoordinates, endCoordinates - An object containing keys height, width, screenX, and

screenY. These refer to the start and end coordinates of the keyboard. Normally height, width,

and screenX will stay the same. We can use height to determine the height of the keyboard. The

screenY value refers to the top of the keyboard, which we can use to determine the remaining

height available to render content.

To calculate the contentHeight, we can take the screenY (top coordinate of the keyboard) and

subtract layout.y (top coordinate of our messaging component).

Core APIs, Part 2 297

1 measure = (event) => {

2 const { layout } = this.props;

4 const { endCoordinates: { height, screenY }, duration = INITIAL_ANIMATION_DURATION\

5 } = event;

7 this.setState({

8 contentHeight: screenY - layout.y,

9 keyboardHeight: height,

10 keyboardAnimationDuration: duration,

11 });

12 };

14 // ...

Remember, y coordinates lower down on the screen are larger than those higher on the

screen, so this calculation will resulting in a positive value.

Note that if a hardware keyboard is connected, the height of the keyboard will be 0 – we’ll have to

handle this specially later.

Let’s propagate all of these values into the children of this component. We’ll also propagate the

height of the entire component as containerHeight.

messaging/components/KeyboardState.js

// ...

render() {

const { children, layout } = this.props;

const {

contentHeight,

keyboardHeight,

keyboardVisible,

keyboardWillShow,

keyboardWillHide,

keyboardAnimationDuration,

} = this.state;

return children({

containerHeight: layout.height,

contentHeight,

keyboardHeight,

keyboardVisible,

Core APIs, Part 2 298

keyboardWillShow,

keyboardWillHide,

keyboardAnimationDuration,

});

}

// ...

When we use this component, it’ll look roughly like this: we’ll first wrap it in our MeasureLayout

component, and pass the layout in as a prop. We can then render our content using the keyboardInfo

object.

{layout => (

{keyboardInfo => /* ... */}

</KeyboardState>

)}

</MeasureLayout>

Alright, we’re almost there! We have MeasureLayout and KeyboardState. The last component we

need is MessagingContainer to render the content using the sizes we’ve calculated.

MessagingContainer

Let’s create a new component MessagingContainer to render the correct Input Method Editor (IME)

at the correct size.

Once again, let’s figure out the propTypes first. This component is going to have a lot of props, since

it’s consuming data from the previous components we wrote, in addition to more props which we’ll

pass in from App.

The main job of this component is to display the correct IME at any given time. Let’s define constants

for each potential state:

• NONE - Don’t show any IME.

• KEYBOARD - The text input is focused, so the keyboard should be visible.

• CUSTOM - Show our custom IME. In this case, we’ll show our image picker, but we could show

other kinds of input here if we wanted to.

Let’s create an object to hold these. We’ll also export it so that other components can easily use the

correct string values.

Core APIs, Part 2 299

messaging/components/MessagingContainer.js

import { BackHandler, LayoutAnimation, Platform, UIManager, View } from 'react-nativ\

e';

import PropTypes from 'prop-types';

import React from 'react';

export const INPUT_METHOD = {

NONE: 'NONE',

KEYBOARD: 'KEYBOARD',

CUSTOM: 'CUSTOM',

};

// ...

Now for the propTypes. We’ll begin by declaring each of the values that will be passed from

KeyboardState. We’ll define inputMethod and onChangeInputMethod to handle switching between

IMEs and notifying the parent of changes. We’ll also support rendering content in both the keyboard

area with renderInputMethodEditor and the main content area with children. In this case, children

should be a normal React element rather than a function like in the two components we wrote

previously.

messaging/components/MessagingContainer.js

// ...

export default class MessagingContainer extends React.Component {

static propTypes = {

// From `KeyboardState`

containerHeight: PropTypes.number.isRequired,

contentHeight: PropTypes.number.isRequired,

keyboardHeight: PropTypes.number.isRequired,

keyboardVisible: PropTypes.bool.isRequired,

keyboardWillShow: PropTypes.bool.isRequired,

keyboardWillHide: PropTypes.bool.isRequired,

keyboardAnimationDuration: PropTypes.number.isRequired,

// Managing the IME type

inputMethod: PropTypes.oneOf(Object.values(INPUT_METHOD)).isRequired,

onChangeInputMethod: PropTypes.func,

// Rendering content

children: PropTypes.node,

renderInputMethodEditor: PropTypes.func.isRequired,

Core APIs, Part 2 300

};

static defaultProps = {

children: null,

onChangeInputMethod: () => {},

};

// ...

}

Now let’s use componentWillReceiveProps to handle switching the inputMethod. When the key-

board transitions from hidden to visible, we want to set the inputMethod to INPUT_METHOD.KEYBOARD.

When the keyboard transitions from visible to hidden, we want to set the inputMethod to INPUT_-

METHOD.NONE… unless we’re currently displaying the image picker (the keyboard should always be

hidden when we display the image picker, so we can ignore this transition).

messaging/components/MessagingContainer.js

// ...

componentWillReceiveProps(nextProps) {

const { onChangeInputMethod } = this.props;

if (!this.props.keyboardVisible && nextProps.keyboardVisible) {

// Keyboard shown

onChangeInputMethod(INPUT_METHOD.KEYBOARD);

} else if (

// Keyboard hidden

this.props.keyboardVisible &&

!nextProps.keyboardVisible &&

this.props.inputMethod !== INPUT_METHOD.CUSTOM

) {

onChangeInputMethod(INPUT_METHOD.NONE);

}

// ... more to come!

}

// ...

Since inputMethod will be stored in the state of the parent, we’ll call onChangeInputMethod

and let the parent pass this prop back down. We could store inputMethod in the state of

Core APIs, Part 2 301

MessagingContainer, but since the parent needs to access this value, it’s best that the parent stores

it.

LayoutAnimation

We’re going to use LayoutAnimation to handle automatically transitioning between the various

states of this component. LayoutAnimation is still considered experimental, and it’s more common

to use the other animated API, Animated. However, LayoutAnimation is the only way we can match

the exact animation of the keyboard. It’s used internally by the built-in KeyboardAvoidingView

component, so it’s safe for us to use despite being considered experimental.

Currently LayoutAnimation is disabled by default on Android, so we need to enable it by calling

UIManager.setLayoutAnimationEnabledExperimental(true). We can enable it anywhere in the app,

but let’s do it at the top of MessagingContainer.js, since that’s the file we use it in:

messaging/components/MessagingContainer.js

if (Platform.OS === 'android' && UIManager.setLayoutAnimationEnabledExperimental) {

UIManager.setLayoutAnimationEnabledExperimental(true);

}

The UIManager object contains a variety of APIs for getting access to native UI elements

for measuring, but we won’t use it for anything else here.

LayoutAnimation automatically handles animating elements that should change size or appear/dis-

appear between calls to render. We call LayoutAnimation.create to define an animation configu-

ration, and then LayoutAnimation.configureNext to enqueue the animation to run the next time

render is called.

The LayoutAnimation.create API takes three parameters:

• duration - The duration of the animation

• easing - The curve of the animation. We choose from a predefined set of curves: spring, linear,

easeInEaseOut, easeIn, easeOut, keyboard. The keyboard curve is the key to matching the

keyboard’s animation curve – although it only exists on iOS.

• creationProp - The style to animate when a new element is added: opacity or scaleXY.

In our case, we want to call this every time the component will re-render, so componentWillReceiveProps

is the best place.

Core APIs, Part 2 302

messaging/components/MessagingContainer.js

// ...

componentWillReceiveProps(nextProps) {

// ... from before!

const { keyboardAnimationDuration } = nextProps;

const animation = LayoutAnimation.create(

keyboardAnimationDuration,

Platform.OS === 'android' ? LayoutAnimation.Types.easeInEaseOut : LayoutAnimatio\

n.Types.keyboard,

LayoutAnimation.Properties.opacity,

);

LayoutAnimation.configureNext(animation);

}

// ...

LayoutAnimation applies to the entire component hierarchy, not just the component we call it from,

so this will actually animate every component in our app. It may be a better idea to selectively choose

when to animate based on the exact props which have changed, but for simplicity, let’s assume we

always want to animate.

Handling the back button

We should add one last bit of logic in MessagingContainer.js to handle the hardware back button

on Android. When the CUSTOM IME is active, we want the back button to dismiss the IME, just like it

would for the device keyboard. We’ll use BackHandler for this. When the back button is pressed, if

the CUSTOM IME is active, we’ll call onChangeInputMethod(INPUT_METHOD.NONE) to notify the parent.

messaging/components/MessagingContainer.js

// ...

componentDidMount() {

this.subscription = BackHandler.addEventListener('hardwareBackPress', () => {

const { onChangeInputMethod, inputMethod } = this.props;

if (inputMethod === INPUT_METHOD.CUSTOM) {

onChangeInputMethod(INPUT_METHOD.NONE);

return true;

Core APIs, Part 2 303

}

return false;

});

}

componentWillUnmount() {

this.subscription.remove();

}

// ...

Rendering the MessagingContainer

Now let’s render this thing! We’ll render an outer View which contains the message list and the tool-

bar (via children), and an inner View which renders the image picker (via renderInputMethodEditor).

The conditional logic is pretty complex, so let’s take a look at it in-line with the code.

messaging/components/MessagingContainer.js

// ...

render() {

const {

children,

renderInputMethodEditor,

inputMethod,

containerHeight,

contentHeight,

keyboardHeight,

keyboardWillShow,

keyboardWillHide,

} = this.props;

// For our outer `View`, we want to choose between rendering at full

// height (`containerHeight`) or only the height above the keyboard

// (`contentHeight`). If the keyboard is currently appearing

// (`keyboardWillShow` is `true`) or if it's fully visible

// (`inputMethod === INPUT_METHOD.KEYBOARD`), we should use

// `contentHeight`.

const useContentHeight =

keyboardWillShow || inputMethod === INPUT_METHOD.KEYBOARD;

Core APIs, Part 2 304

const containerStyle = {

height: useContentHeight ? contentHeight : containerHeight,

};

// We want to render our custom input when the user has pressed the camera

// button (`inputMethod === INPUT_METHOD.CUSTOM`), so long as the keyboard

// isn't currently appearing (which would mean the input field has received

// focus, but we haven't updated the `inputMethod` yet).

const showCustomInput =

inputMethod === INPUT_METHOD.CUSTOM && !keyboardWillShow;

// If `keyboardHeight` is `0`, this means a hardware keyboard is connected

// to the device. We still want to show our custom image picker when a

// hardware keyboard is connected, so let's set `keyboardHeight` to `250`

// in this case.

const inputStyle = {

height: showCustomInput ? keyboardHeight || 250 : 0,

};

return (

{children}

<View style={inputStyle}>{renderInputMethodEditor()}</View>

</View>

);

}

// ...

In order for the toolbar to sit above the home indicator on the iPhone X, we’ll need to adjust the

space below the toolbar as the keyboard transitions up and down.

Supporting the iPhone X

You may skip this section if you’re not testing with an iPhone X.

In the “Core Components” chapter, we used the SafeAreaView to support the iPhone X. This won’t

work here, since we want to animate the space below the toolbar (to avoid a jerk when the space

changes).

Core APIs, Part 2 305

We’ll install the npm library react-native-iphone-x-helper

to help us determine if the device is

an iPhone X.

In your terminal, install the library with:

yarn add react-native-iphone-x-helper@1.0.1

React Native doesn’t currently provide a way to determine if the device is an iPhone X. This

library simply checks the device’s dimensions. Hopefully in the future something better will

be provided out-of-the-box for accessing the safe area insets directly.

After this finishes, import the isIphoneX utility function at the top of the file:

messaging/components/MessagingContainer.js

import { isIphoneX } from 'react-native-iphone-x-helper';

Now we can update the render method above to include extra space below the toolbar:

messaging/components/MessagingContainer.js

// ...

render() {

// ...

// The keyboard is hidden and not transitioning up

const keyboardIsHidden =

inputMethod === INPUT_METHOD.NONE && !keyboardWillShow;

// The keyboard is visible and transitioning down

const keyboardIsHiding =

inputMethod === INPUT_METHOD.KEYBOARD && keyboardWillHide;

const inputStyle = {

height: showCustomInput ? keyboardHeight || 250 : 0,

// Show extra space if the device is an iPhone X the keyboard is not visible

marginTop: isIphoneX() && (keyboardIsHidden || keyboardIsHiding) ? 24 : 0,

};

// ...

https://www.npmjs.com/package/react-native-iphone-x-helper

Core APIs, Part 2 306

}

// ...

Whew, we made it. Save MessagingContainer.js. Now we just need to render MessagingContainer

from App.

Rendering MessagingContainer in App

Head back to App.js and import the components we’ve just created:

messaging/App.js

// ...

import KeyboardState from './components/KeyboardState';

import MeasureLayout from './components/MeasureLayout';

import MessagingContainer, { INPUT_METHOD } from './components/MessagingContainer';

// ...

Let’s include the inputMethod in the state of App, and handle changes to it.

messaging/App.js

// ...

export default class App extends React.Component {

state = {

// ...

inputMethod: INPUT_METHOD.NONE,

};

// ...

handleChangeInputMethod = (inputMethod) => {

this.setState({ inputMethod });

};

handlePressToolbarCamera = () => {

this.setState({

isInputFocused: false,

inputMethod: INPUT_METHOD.CUSTOM,

});

Core APIs, Part 2 307

};

// ...

}

// ...

Lastly, let’s use MeasureLayout, KeyboardState, and MessagingContainer to render the UI compo-

nents we’ve already written. We’ll rearrange this.renderMessageList,this.renderToolbar, and

this.renderInputMethodEditor so that they render within MessagingContainer.

We can update the render method of App to look like this:

// ...

render() {

const { inputMethod } = this.state;

return (

{layout => (

{keyboardInfo => (

<MessagingContainer

{...keyboardInfo}

inputMethod={inputMethod}

onChangeInputMethod={this.handleChangeInputMethod}

renderInputMethodEditor={this.renderInputMethodEditor}

{this.renderMessageList()}

{this.renderToolbar()}

</MessagingContainer>

)}

</KeyboardState>

)}

</MeasureLayout>

{this.renderFullscreenImage()}

</View>

);

}

Core APIs, Part 2 308

// ...

Using the children function prop pattern, we can pretty clearly visualize the flow of data downward

into MessagingContainer.

Note that since keyboardInfo contains many properties, it’s easiest to pass them all into MessagingContainer

at once with the object spread syntax ...keyboardInfo. If we prefer, we could also assign each

property individually, e.g. keyboardHeight={keyboardInfo.keyboardHeight}.

Save App.js and test it out! You should see the same components as before, but now they animate

smoothly to avoid the keyboard as it appears and disappears.

In the default state, our app should look like this:

Tapping the input field should pop open the keyboard, and smoothly transition the rest of the UI:

Core APIs, Part 2 309

Tapping the camera icon should transition to the image picker:

Core APIs, Part 2 310

We’re Done!

We’ve built a messaging app UI complete with text messages, images, and maps. We notify the

user of connectivity issues. We display a pixel-perfect infinite scrolling grid of photos. We smoothly

animate the UI as new messages are added and removed (try it if you haven’t! the LayoutAnimation

takes care of this automatically). We handle the keyboard gracefully on both platforms.

Navigation

In the “Core Components” chapter, we explored how different parts of the app (the image feed and

user comments) can be represented as separate screens - components that take up the entire device

screen. When building screens, handling how the user navigates between them is a primary concern.

Navigation is a major piece of any mobile application with multiple screens. With a navigation

system in place, a user can access any part of an application. It also allows us to structure and

separate how data is handled in the app.

Handling navigation in a mobile application is fundamentally different from a website. For a website,

the state of a user’s location is usually kept in the browser’s URL. Although the browser maintains

a history of pages visited in order to allow the user to move back and forth, the browser only stores

page URLs and is otherwise stateless. On mobile, the entire history stack is maintained and can be

accessed.

On mobile, we have more control and flexibility over history management. We can keep a history

stack that includes details of each route including parameters and part of the application state.

Further, mobile navigation presents its own set of challenges. One of the biggest is the reduced real

estate of the user’s device screen compared to a desktop or laptop computer. We need to make sure

there are easily visible and identifiable navigation components that will allow the user to move to

another part of the application when pressed. Including a complex navigation flow comes with the

cost of a larger number of navigation components (such as menu options). For this reason, most

mobile apps tend to have a small and focused number of screens that a user can easily navigate to

and understand.

Navigation in React Native

This section will explore the landscape of navigation in React Native in some detail. If you

would like to jump straight to building our sample application, feel free to skip this section

and return to it later.

One of the primary navigation patterns in a mobile app is a stack-based pattern. In this pattern, only

one screen can be seen by the user at any given time. Navigating involves pushing the new screen

onto the navigation stack. We’ll explore stack-based navigation in more detail later in the chapter.

For now, it is important to realize that this pattern, among others, uses different native components

for iOS and Android. For example, building a stack-based navigation flow between screens can be

done using UINavigationController

for iOS and connecting Activities

for Android.

https://developer.apple.com/documentation/uikit/uinavigationcontroller

https://developer.android.com/guide/components/activities/index.html

Navigation 312

There are two primary approaches to navigation in React. We can either include actual native

iOS/Android navigational elements or use JavaScript to create the required animations and com-

ponents that we need.

Native navigation

The first way we can add navigation is to use native iOS/Android navigational components.

In an iOS application, views are used to build the UI and display content to the user. A view controller

(or the UIViewController class) is used to control a set of views and allows us to connect our UI

with our application data. By including multiple view controllers in our app, we can build different

screens as well as transition between them.

A navigation controller (UINavigationController ) simplifies the process of navigating between

screens by allowing us to pass in a stack of UIViewControllers. It will take care of including a

header navigation bar at the top of our device with a back button that allows us to pop the current

view controller off of the current stack. With this, it maintains the hierarchy of all the screens within

the stack.

Example of a navigation controller (from Apple Developer Documentation - UINavigationController)

In Android, activities are used to create single screens to define our UI. We can use tasks in order

to define a stack of activities known as the back stack. The startActivity method can be used to

start a new activity. When this happens, the activity is pushed onto the activity stack. In order to

return to the previous screen, the physical back button on every Android device can be pressed in

order to run the finish method on the activity. This closes the current activity, pops it off the stack

and returns the user back to the previous activity.

Navigation 313

Android Back Stack (from Android Developers Documentation - Tasks and Back Stack)

In React Native, all of our component code executes on a JavaScript thread. These components then

bridge to a separate main thread responsible for rendering native iOS and Android views.

In the first chapter, we briefly mentioned how we can eject from Expo if we need to include any

native dependencies ourselves. This includes any native iOS or Android code we wish to write

ourselves or third-party libraries that provide a React Native API which bridges to a specific native

module. We can use this to include native navigation in our application. We can create native

modules around platform-specific navigation components (such as UINavigationController and

Activity) and bridge them ourselves in a React Native app.

We’ll explore bridging native APIs in much more detail in the “Native Modules” chapter.

Pros

The primary benefit of this approach is a smoother navigation experience for the user. This is because

purely native iOS/Android navigation APIs can be used with all of our navigation happening within

the native thread. This approach works well when including React Native in an existing native iOS

or Android application. Using the same navigation components and transitions throughout the app

means that different screens in the app will feel consistent regardless of whether they’re written

natively or with React Native.

Additionally, if an operating system update modifies the style or functionality of navigation

components, you won’t have to wait for the same modifications to be made in your JavaScript-

based navigation library.

Cons

One of the issues with this navigation approach is that it usually involves more work. This is

because we need to ensure navigation works for both iOS and Android, using their respective native

navigational components.

Navigation 314

Moreover, we will also have to eject from Expo and take care of linking and bridging any

native modules ourselves. This means we cannot build an application with native iOS navigation

components if we do not own a Mac computer.

Another potential problem with this solution is that it can be significantly harder to modify or create

new navigation patterns. In order to customize how navigation is performed, we would have to dive

in to the native code and understand how the underlying navigation APIs work before being able

to change them.

Navigation with JavaScript

The second approach to adding navigation to a React Native app is to use JavaScript to create

components and navigation patterns that look and feel like their native counterparts. This is done

solely using React Native built-in components and the Animated API for animations.

We can explain how this works by using stack navigation as an example again. As we mentioned

earlier, this pattern allows us to move between screens by pushing a second screen on top of the

previous one. We usually see this happen by seeing the second screen slide in from either the right

or bottom edge of the device screen or fading in from the bottom. When we attempt to navigate

backwards, the current screen slides back out in the opposite direction or fades out from the top.

With the Animated API, we can use animated versions of some built-in components such as View as

well as create our own. We can create stack based navigation (as well as other navigation patterns)

by nesting our screen components within an Animated component. We can then have our screens

slide (or fade) in and out of our device when we need to allow the user to navigate throughout our

application. We’ll have to maintain the hierarchy of screens entirely in JavaScript ourselves.

Pros

One of the advantages of using JavaScript-based navigation is that it can be simpler to build the

components and animation mechanisms that can be used in both platforms instead of trying to

create a bridge to all of the core native iOS/Android APIs that we would need. This also gives us

more control and flexibility to customize specific navigation features instead of relying on what’s

available in the native platforms. We can debug any issues we experience with navigation that is

purely JavaScript-based without diving in to native code.

Most of the work during an animation using the React Native Animated API is on the JavaScript

thread. This means that every frame needs to go over the bridge to the native thread to update the

views during a transition. Fortunately, we have the option to use the API’s native driver option to

render natively driven animations. These animations are performed with animation calculations

happening on the native thread. By building navigation with this, navigation animations will

perform smoothly.

We’ll explore the built-in Animated API in greater detail in the next chapter.

Navigation 315

Another benefit of keeping all of our navigation elements within the JavaScript thread means that

we can take advantage of services such as CodePush

to allow us to dynamically update the

application’s JavaScript code (which includes our navigation) without rolling out a new build to

our users.

Cons

There are also disadvantages with this approach. Firstly, the app can never feel exactly like a native

application in terms of navigation. As much as we can try to mimic how navigation components

and animations look like in the native layer, there may always be slight discrepancies. This can be

a bigger problem if we happen to be including React Native components into an existing native iOS

or Android application. Building transitions between screens built natively and screens built with

React Native can be a challenge if we’re using only JavaScript for our navigation.

Another potential concern with JavaScript-based navigation is slower updates in relation to the

underlying platform’s operating system. Updates to the iOS or Android version may bring changes

to the native navigational views and components. We’ll need to make sure the views and components

built with JavaScript are also updated in order to better match how they are represented natively.

Third-party libraries

In React Native, we have the option of setting up navigation by creating our own native modules

or by building our own JavaScript-based implementation. There are also a number of community-

supported libraries that we can use for either of these approaches:

• React Native Navigation

by Wix engineering and Native Navigation

by Airbnb are both

navigation libraries that provide access to native iOS/Android navigation components using a

React Native API.

• React Navigation

and React Router

are two popular third-party JavaScript navigation

libraries.

Using one of these libraries can make things significantly easier than building a navigation pattern

entirely from scratch. Moreover, all of these community-built libraries are continuously maintained

with updates included in each new release.

Navigation alternatives

Not all mobile applications need to have a complete navigation architecture. Examples include an

app that only has a few screens or does not even need any navigation in the first place (such as a

https://github.com/Microsoft/react-native-code-push

https://wix.github.io/react-native-navigation/#/

http://airbnb.io/native-navigation/

https://facebook.github.io/react-native/docs/navigation.html#react-navigation

https://reacttraining.com/react-router/native/guides/philosophy

Navigation 316

single-screen game). However, most applications with more than a few screens will usually need

some form of navigation to allow the user to move between them.

There is no single correct solution for applications that require navigation. Different mobile apps will

always have different features and complexities. For example, it may be easier to use a JavaScript

implementation in a brand new and relatively simple application without complex navigation

requirements. However, we might find it easier to use a native solution if we plan on rolling React

Native components into a native application. We should always weigh the benefits and challenges

of each solution before deciding which approach to take.

Deprecated solutions

Navigation is a core tenet of any native application. Just like other built-in components (such as View

and Text), React Native also used to provide a number of different built-in navigation APIs. Here

are a few examples:

• NavigatorIOS

provides an API to access the UINavigationController component for iOS to

build a screen navigation stack. It is not currently maintained and cannot be used for Android.

• Navigator is a JavaScript navigation implementation that was included into React Native when

it first launched. Expo built ExNavigator on top of this API with the aim of providing more.

features. However, it did not provide a complete navigation solution and was deprecated soon

after.

• NavigationExperimental is another JavaScript implementation and aimed to solve the prob-

lems noticed in Navigator. ExNavigation was built by the Expo team to act as a wrapper

around NavigationExperimental. It is also now deprecated.

It is important to note that all of these APIs are either not maintained or are deprecated. It is

recommended to use one of the newer community-built navigation libraries instead.

Since navigation is an important part of many mobile applications, all of these efforts were done in

order to provide a simple React Native API that can be imported and used directly in a component.

However, navigation is a lot more complex than many other built-in components. It is not easy to

provide a simple navigation API that can solve all navigation concerns in any application. For this

reason, a number of different open-source alternatives were created by the community. The efforts

from Navigator, NavigationExperimental, and the community-built ex-navigation were combined

to form the community-built React Navigation library.

https://facebook.github.io/react-native/docs/navigatorios.html

In this chapter

We covered the differences between native and JavaScript navigation implementations as well

as some of their advantages and disadvantages. For each approach, we also discussed how using

an open-source library that is continuously maintained can make things easier than building a

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navigation architecture from scratch.

Co-authored by the Facebook team and the open-source community, React Navigation

is the

recommended option in the React Native documentation. For this reason, we’ll use React Navigation,

a JavaScript-based implementation, in this chapter to handle navigation in our sample application.

Contact List

In this chapter, we’re going to build a contact list application that allows a user to view contact

information across several screens. We’ll begin by building our first screen, Contacts, that shows a

list of contact information fetched from a remote API.

Contacts

Then we’ll explore how we can allow the user to navigate to a separate Profile screen for each

specific contact.

https://facebook.github.io/react-native/docs/navigation.html#react-navigation

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Profile

We’ll then include another top level screen, Favorites, to show favorited contacts.

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Favorites

Then we’ll build a User screen and tie together our top level screens using a tab navigation

component.

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User

The last screen we’ll create will be an options screen that the user can navigate to through the user

screen.

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Options

By building out each screen and connecting them, we’ll get a better understanding of how a

navigation pattern such as tabs can be coupled with stack navigation. While doing so, we’ll also look

into creating a small state container that controls the entire state of our app and can be accessed in

any of our screens.

By the time we’re finished with this chapter, we will have covered a number of different navigation

patterns and show how they can fit together. We’ll also be touching on a number of core concepts

we’ve already covered in the previous chapters.

Previewing the app

Before we begin, to try the completed app on your device:

• On Android, you can scan this QR code with the Expo app:

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QR Code

• On iOS, you can navigate to the contact-list/ directory within the sample code folder and

either preview it on the iOS simulator or send the link of the project URL to your device as we

explained in the first chapter.

Spend a little time navigating between the different screens to get a feel for all the functionality.

Starting the project

Just as we did in the previous chapters, let’s create a new app with the following command:

create-react-native-app contact-list --scripts-version 1.14.0

Once this finishes, navigate into the contact-list directory and start the app.

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Like you did in previous chapters, copy over the

contact-list/utils

directory from the sample

code into your own project. The utils/ directory contains the following:

• A few utility methods

• Methods that return results from our external API, Random User Generator

• A colors object with a number of different colors

Copy over the contact-list/components directory as well. These components are the low level

presentational components that don’t manage any state of their own. They are used in the app to

display UI elements in all of our screens. Here’s a brief overview of each of the components:

• ContactListItem will be used for each contact’s list item in the Contacts screen.

• ContactThumbnail renders a thumbnail for the contact avatar that can fire an action when

pressed. It can also show the user’s name and phone number underneath based on optional

props. This component will be used to render a list of user avatars in the Favorites screen as

well as show the user thumbnail in the Profile and User screens.

• DetailListItem

shows a list item with a title, subtitle, and an optional icon. This component

will be used to show contact details in the Profile screen as well as mocked links in the Options

screen.

https://randomuser.me/

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Feel free to dive in and take a closer look at any of the files within utils/ and components/ to get a

better idea of how they work.

Container and Presentational components

Before we dive in to building our application, let’s take a little time to further understand how

we separate our screen and component logic. In all of our previous chapters, we explored building

custom components to create higher-level abstractions over built-in components (such as View, Text,

etc…). We can think of screens in the same way. Just like any other component, screens wrap over

lower level components. The difference here is that we can build our screen components to take up

the entire device screen and allow the user to navigate between them.

We briefly explored this pattern in the “Core Components” chapter where we built a Feed and

Comments screen for our Instagram clone app. While doing so, we managed all of our remote data

fetching within the Feed screen and the rest of our lower level components only received this

information via props. This further ties in to the pattern we’ve seen in each of the applications

we’ve built in this book so far: the concept of container components that take care of data fetching

and state management and presentational components that take in data and provide the markup

and styling in our application. We can closely follow this logic by separating how we build screens

and components in an application.

Although the Feed screen was responsible for data fetching in our Instagram clone app, we still had

some state managed in our root App component. For a relatively large application with a significant

number of screens, managing data in a single component like App may not be the most maintainable

way to handle state. For this reason, third-party state container libraries are commonly used. Instead

of using a specific library and trying to understand its APIs, we’ll handle data in our application by

writing a small custom state container. The same pattern will apply to any complex application with

a central state container regardless of which library is used.

Contacts

The first screen we’ll build is the main contacts screen which will also serve as the starting point

of our application. Create a screens/ directory and add a Contacts.js file. As we mentioned in

the “Core Components” chapter, a more complex application might be structured with directories

nested within screens/ for better categorizing of files. Since this application only consists of five

screens, we’ll add them all to the screens/ directory.

We’ll begin by defining our imports in the file:

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contact-list/1/screens/Contacts.js

import React from 'react';

import {

StyleSheet,

Text,

View,

FlatList,

ActivityIndicator,

} from 'react-native';

import ContactListItem from '../components/ContactListItem';

import { fetchContacts } from '../utils/api';

We’ve imported a few necessary built-in components including FlatList and ActivityIndicator

as well as our custom ContactListItem component responsible for displaying each of our contacts

items in the list. Aside from components, we also import the fetchContacts method in order to

retrieve our list of contacts and our colors object from utils.

Now let’s begin creating our class component:

contact-list/1/screens/Contacts.js

export default class Contacts extends React.Component {

state = {

contacts: [],

loading: true,

error: false,

};

async componentDidMount() {

try {

const contacts = await fetchContacts();

this.setState({

contacts,

loading: false,

error: false,

});

} catch (e) {

this.setState({

loading: false,

error: true,

});

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}

We’ve set up local component state that includes a contacts array and loading/error attributes.

In here, we’ve set our initial loading property to true because we fire our API call as soon as our

component mounts. We then update this to false as soon as our request finishes successfully.

Let’s now build the UI that gets rendered on the screen:

contact-list/1/screens/Contacts.js

renderContact = ({ item }) => {

const { name, avatar, phone } = item;

return <ContactListItem name={name} avatar={avatar} phone={phone} />;

};

render() {

const { loading, contacts, error } = this.state;

const contactsSorted = contacts.sort((a, b) =>

a.name.localeCompare(b.name));

return (

{loading && <ActivityIndicator size="large" />}

{error && <Text>Error...</Text>}

{!loading &&

!error && (

<FlatList

data={contactsSorted}

keyExtractor={keyExtractor}

renderItem={this.renderContact}

)}

</View>

);

}

In the component render method, we sort our contacts alphabetically and show a loading indicator

if state.loading is true, an error message if state.error is true, or a list of our contacts using

FlatList. For each item in the list, we use a renderContact helper method that passes down the

contact name, avatar and phone as props to ContactListItem.

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We’ll also need to create our list’s keyExtractor method which we can write under our imports at

the top of the file:

contact-list/1/screens/Contacts.js

import { fetchContacts } from '../utils/api';

const keyExtractor = ({ phone }) => phone;

export default class Contacts extends React.Component {

The last thing we’ll need to do is set up the styles for this component. Since our existing

presentational component ContactListItem takes care of most of our styling, we’ll just set up styles

for the container View component:

contact-list/1/screens/Contacts.js

const styles = StyleSheet.create({

container: {

backgroundColor: 'white',

justifyContent: 'center',

flex: 1,

});

Try it out

To quickly take a look at how this screen renders, we can temporarily place this component within

App:

contact-list/App.js

import React from 'react';

import Contacts from './screens/Contacts';

export default function App() {

return <Contacts />;

}

Now we can see our Contacts screen if we run the app:

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Contacts

You may notice the top and bottom of our list touches the edges of our device screen. This will be

fixed once we introduce our header and tab navigation components to the app. Pressing any of the

contacts does not do anything just yet. We’ll explore how we can navigate to a specific contact’s

profile screen in a bit.

You may also see a warning about a missing onPress prop which is required in the ContactListItem

component. We’ll include it once we begin adding navigation to our application.

Profile

Let’s move on to building our second screen, Profile, which shows details about a specific contact.

Create a Profile.js file within the same screens directory. Again, we’ll begin with our imports:

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contact-list/1/screens/Profile.js

import React from 'react';

import { StyleSheet, View } from 'react-native';

import ContactThumbnail from '../components/ContactThumbnail';

import DetailListItem from '../components/DetailListItem';

import { fetchRandomContact } from '../utils/api';

import colors from '../utils/colors';

We’ve included the ContactThumbnail and DetailListItem presentational components that we’ll

need for this screen as well as the colors object we’ll use for some styling.

We’ve also included fetchRandomContact to obtain a random contact’s information. This is tempo-

rary in order to render this screen for the first time, but will be removed once we have navigation

in place and the contact ID is passed from the previous screen to the Profile screen.

We can build our class component as follows:

contact-list/1/screens/Profile.js

export default class Profile extends React.Component {

state = {

contact: {},

};

async componentDidMount() {

const contact = await fetchRandomContact();

this.setState({

contact,

});

}

render() {

const {

avatar, name, email, phone, cell,

} = this.state.contact;

return (

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</View>

</View>

);

}

We’ve defined a contact object as our only attribute in our component state. Our componentDidMount

method fires an API call to get a random contact. Again, this is temporary until we’ve included our

navigation library. This is because we’ll eventually pass the contact’s information from the Contacts

screen.

We have not included any loading or error attributes for this same reason. This is because once

navigation is in place, this screen will only be accessible through another screen by pressing on a

contact list item or thumbnail. This means there will be no loading or potential errors from data

fetching happening at this point.

Although this might usually be the case when building screens that are only accessible through

other screens in a stack, there are scenarios where we may need to fetch data in nested screens. A

good example is deep linking which allows a user to navigate to a certain part of the app through

another app or a web browser using a specific link. We’ll explore this topic later in this chapter.

The render method is relatively straightforward. We show the user thumbnail for the top half of the

screen using ContactThumbnail as our component (which accepts the user’s avatar, name, and phone

as props). For the bottom half of the screen, we’re displaying a few DetailListItem components to

show the user’s email, work, and cell numbers.

We can now create styling for the View container components we’re using for layout at the end of

the file:

contact-list/1/screens/Profile.js

const styles = StyleSheet.create({

container: {

flex: 1,

avatarSection: {

flex: 1,

alignItems: 'center',

justifyContent: 'center',

backgroundColor: colors.blue,

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detailsSection: {

flex: 1,

backgroundColor: 'white',

});

Try it out

Once again, let’s render our screen to see if everything is working well:

contact-list/App.js

import React from 'react';

import Profile from './screens/Profile';

export default function App() {

return <Profile />;

}

Running the app should show the Profile screen for a random contact:

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Profile

React Navigation

Now that we have our first two screens in place, let’s start adding navigation to our app! As we

mentioned earlier in the chapter, there are a number of different open-source navigation libraries

available. We’ll be using React Navigation

for this application. Since it’s purely a JavaScript

implementation, we don’t have to worry about linking iOS and Android dependencies. We can

install it to our app by using yarn:

yarn add react-navigation@1.5.11

Specify version 1.5.11 as above so that the version in your application matches the version we use

here.

Stack navigation

We briefly described how a stack navigator works earlier in this chapter. This pattern allows a user

to navigate from one screen to another by pushing the new screen to the top of the stack. The user

can also pop the current screen off the stack in order to return to the previous screen.

https://facebook.github.io/react-native/docs/navigation.html#react-navigation

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In both iOS and Android, a back button at the top of the navigation bar is how a user usually

navigates back to a previous screen by removing the current screen off the stack. On Android devices,

there is also a physical or soft key back button at the bottom of the device screen that also allows

you to go back on any application.

With this navigation flow, only one screen is visible at any given time. We can think of the entire

navigation stack as an ordered array of screens, with the last element being the screen that is

currently visible and the first element being the root screen (or the screen that is visible when loading

the app for the first time).

Let’s begin by connecting our first two screens as a single stack. We’ll define all of our navigation

logic in a separate routes.js file at the root of our entire app.

Create the file and add the following code:

contact-list/2/routes.js

import { StackNavigator } from 'react-navigation';

import Contacts from './screens/Contacts';

import Profile from './screens/Profile';

export default StackNavigator({

Contacts: {

screen: Contacts,

Profile: {

screen: Profile,

});

We pass in our route configuration as an argument to StackNavigator. The object maps route names

to their configuration. We have two routes defined above, Contacts and Profile.

For each route, we define a configuration object with just one property: screen. This is the

component we wish to render at that specific route. We’ll explore more configuration options in

a bit.

We can also pass a second argument to StackNavigator, our stack navigator configurations. We’ll

also explore this a little later in this section.

In React Navigation, every navigator including StackNavigator creates a higher-order component

that wraps over each of the screen components defined within its route configurations. It enhances

each of its components by creating a newer component with a navigation prop.

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Higher-Order Components

In short, higher-order components are functions that take in an existing component and

return a new component with added functionality. They’re useful for minimizing code

duplication by containing common logic in a single component that can be shared among

multiple components. They’re also useful for libraries like React Navigation.

Internally, StackNavigator() generates a higher-order component that provides each of

our screens with a navigation prop. This prop serves as the interface between our screen

components and the React Navigation library.

For more information on higher-order components, refer to its section in the Appendix.

The navigation prop provides us with the following:

• navigate: Method to allow us to navigate between screens. With a StackNavigator, this

method pushes the new screen on top of the current stack.

• state: Object that returns the name and identifier of the current route as well as its parameters.

• setParams: Method to change the current screen’s parameters.

• goBack: Method that allows us to navigate to a previous screen. For StackNavigator, this pops

the current screen (or number of screens) until the specified screen is reached within the stack.

The React Navigation documentation

contains more detail about the navigation prop.

Let’s now modify our App.js file to render our navigator instead of a single component:

contact-list/2/App.js

1 import React from 'react';

3 import AppNavigator from './routes';

5 export default function App() {

6 return <AppNavigator />;

7 }

Now that we’ve set up the first StackNavigator of our application, we’ll need to use our navigation

prop to allow the user to navigate from the Contacts screen to the Profile screen. We know that

the ContactListItem component contains an onPress prop that fires the action passed to it. Let’s

modify how we render this component within our Contacts screen:

https://reactnavigation.org/docs/navigation-prop.html

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contact-list/2/screens/Contacts.js

renderContact = ({ item }) => {

const { navigation: { navigate } } = this.props;

const { name, avatar, phone } = item;

return (

<ContactListItem

name={name}

avatar={avatar}

phone={phone}

onPress={() => navigate('Profile')}

);

};

Try running the app and you’ll notice a header navigation bar at the top of the screen. In addition

to supplying the navigation prop, the StackNavigator HOC also renders a header above the screen

components it wraps.

Contacts

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Pressing any contact will navigate to the profile screen. The default behaviour for iOS is an animation

that slides the new screen from the right. For Android, the newer screen fades in from the bottom.

Profile

Pressing the back button also pops the profile screen of the stack and returns us to the contacts

screen.

In your terminal, you may be seeing a warning similar to the following:

Warning: isMounted(...) is deprecated in plain JavaScript React classes...

This warning shows when using our current version of React Navigation (1.5.11) with our version

of React Native (0.55.2). There is currently an open GitHub issue

where this should be resolved in

a future release.

https://github.com/react-navigation/react-navigation/issues/3956

Although our stack navigation pattern works, we have two problems:

• Recall that we’re using the fetchRandomContact() method to obtain a random contact. This

means that pressing a specific contact doesn’t actually load their information in the Profile

screen.

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• The header navigation bar doesn’t currently show anything. We should attempt to show the

current screen name so the user knows which screen they’re on.

Navigation parameters

When building multiple screens in a mobile application, it is common to have screens that depend

on some particular data in order to display the correct information. A good example is the Profile

screen in this application. Everytime a user presses a contact on the Contacts screen, we expect to

see that specific contact’s information on the next screen.

The secondary screen here is not a child of the previous screen, but it still relies on a piece of data.

In these scenarios, we need to be able to pass this data as part of the transition in our navigation

flow. React Navigation lets us attach navigation parameters using the navigate method.

We previously mentioned that the navigation prop allows us to change parameters for a screen

using its setParams method. We can similarly pass parameters to another screen. Let’s take a look

at how we set this up for navigating from the Contacts screen to Profile:

contact-list/3/screens/Contacts.js

renderContact = ({ item }) => {

const { navigation: { navigate } } = this.props;

const {

id, name, avatar, phone,

} = item;

return (

<ContactListItem

name={name}

avatar={avatar}

phone={phone}

onPress={() => navigate('Profile', { contact: item })}

);

};

In the second argument of the navigate method, we pass a single object for our parameters that

contains a contact key with the value being the actual contact item. This means that every time we

press a contact on the Contacts screen, the user is navigated to the Profile screen with the actual

contact being passed as a parameter.

With this, we can simplify our Profile screen component and not load a specific contact every time

the screen is mounted:

Since we are now receiving a contact object through navigation props, we no longer need to fetch a

random contact using fetchRandomContact(). We can simplify our Profile screen and not fire any

API calls when our screen mounts:

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contact-list/3/screens/Profile.js

export default class Profile extends React.Component {

render() {

const { navigation: { state: { params } } } = this.props;

const { contact } = params;

const {

avatar, name, email, phone, cell,

} = contact;

return (

</View>

</View>

);

}

We’ve removed all local state from Profile and the component is now driven by props. We extract

the contact object from the navigation prop and use that to render the contact’s information. If

you try running the app now, navigating to a specific contact will show the correct information.

Although passing in the contact object as a navigation parameter works, we generally want to avoid

this pattern.

So far, we’ve explored how parents and children use props to communicate. If a parent performs a

state update, that update propagates through props down to its children. When the child receives

the updated props, the child re-renders.

What’s different here is that Profile is not a direct child of the Contacts screen. Instead Profile is

receiving this data through navigation parameters. Navigation parameters are set once, at the time

of navigation. So if the state for a contact changes, that state update will not be propagated through

navigation parameters.

Put another way, we’ve pushed a copy of a part of our state into navigation parameters. But we have

no means in place to update that copy when the state changes.

So, we should instead pass the id of a contact as a parameter. Then, Profile can look up the contact

in the list of all contacts. We’ll explore this improvement once we introduce a centralized location

for all our state later in this chapter.

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We’ve built the first two screens that make up our first navigator. We covered how to transition

between them by using the API supplied by the navigation prop.

Each screen will usually have its own unique set of features, and we’ll sometimes need to be able to

modify how our navigation-specific components are displayed. Next, we’ll explore how to configure

our navigation screen options before moving on to expanding the number of screens and navigation

patterns in our application.

Navigation screen options

With React Navigation, we can use the navigationOptions property to modify navigation set-

tings for a particular screen or to modify settings for every screen within a navigator (such as

StackNavigator).

We’ll use this property to add a title to the navigation header at the top of both screens. Let’s add it

to each of our screens in route.js:

contact-list/3/routes.js

export default StackNavigator({

Contacts: {

screen: Contacts,

navigationOptions: {

title: 'Contacts',

We add our options using a navigationOptions object. We specify the title attribute to be Contacts.

We can run the application to confirm that this works.

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Contacts

Now let’s do the same for the Profile screen along with adding a few more details:

contact-list/3/routes.js

Profile: {

screen: Profile,

navigationOptions: ({ navigation: { state: { params } } }) => {

const { contact: { name } } = params;

return {

title: name.split(' ')[0],

headerTintColor: 'white',

headerStyle: {

backgroundColor: colors.blue,

};

Although we passed in an object to navigationOptions for the Contacts screen, we also can pass in

a function. Passing a function gives us access to the navigation prop. This is useful when we want

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our options to be derived from the navigation parameters. Here, we get the name of our contact and

use the split

method to only render her or his first name as the title

We also modify the colors of our header by using headerTintColor, which allows us to change

the text color, and headerStyle to pass in an object of styles for the header. We change the

backgroundColor to blue.

If we try navigating to the Profile screen now, we’ll see our header styled appropriately.

Profile

Notice on iOS that the title on the left of the header defaults to the title of the previous

screen. We have control over this with the

headerBackTitle

property of navigation options

if we need to modify this.

For a full list of navigator configuration options provided by StackNavigator, refer to the

documentation

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split

https://reactnavigation.org/docs/navigators/stack

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Default Navigation Options

Although we can specify all of our navigation options for each screen separately, it may

be useful to set default navigation options for multiple screens if they all share the same

configurations. We can do this by defining navigationOptions at the level of the navigator.

For example:

export default StackNavigator({

Contacts: {

screen: Contacts,

navigationOptions: {

headerStyle: {

backgroundColor: 'white',

Profile: {

screen: Profile,

}, {

navigationOptions: {

headerStyle: {

backgroundColor: colors.blue,

});

Screen-specific navigation options for the same configuration will overwrite those defined

for the navigator. In this example, all the screens within this navigator will have a default

background color of blue for their header components. The Contacts screen however will

have a white background color that overwrites the default setting.

Although we can add all of our screen-specific navigation options where we define our navigators

in routes.js, that file can quickly become quite large if we have a lot of screens and configuration

settings. We can instead define each screen’s navigation options inside each component.

Let’s begin with Contacts. We’ll remove the navigationOptions property from our routes.js file

and add it as a static class method to the Contacts component:

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contact-list/4/screens/Contacts.js

export default class Contacts extends React.Component {

static navigationOptions = {

title: 'Contacts',

};

state = {

This is the same technique we’ve used previously for propTypes and defaultProps. We can now do

the same thing for Profile:

contact-list/4/screens/Profile.js

export default class Profile extends React.Component {

static navigationOptions = ({ navigation: { state: { params } } }) => {

const { contact: { name } } = params;

return {

title: name.split(' ')[0],

headerTintColor: 'white',

headerStyle: {

backgroundColor: colors.blue,

};

render() {

Without the navigationOptions properties, we can clean up our routes.js file:

contact-list/4/routes.js

1 import { StackNavigator } from 'react-navigation';

3 import Contacts from './screens/Contacts';

4 import Profile from './screens/Profile';

6 export default StackNavigator(

7 {

8 Contacts: {

9 screen: Contacts,

10 },

11 Profile: {

12 screen: Profile,

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13 },

14 },

15 {

16 initialRouteName: 'Contacts',

17 },

18 );

The only new thing we’ve added here is the initialRouteName property. Without this property,

the first screen listed is the default screen. However, it’s better to explicitly define the initial route,

since we generally shouldn’t rely on the implicit order of keys within an object. Now, wherever this

navigator is loaded in the application, the first screen that shows will be the Contacts screen.

Like initialRouteName, React Navigation allows us to modify a number of different con-

figuration properties for stack navigators besides navigationOptions. The documentation

goes into more detail on each of them.

Tab navigation

A single stack navigator might suffice for a small mobile app with just two or three screens. However,

most applications have more than a few screens and only using stack navigation may not be the most

efficient way to navigate throughout the entire app. This is where another navigation paradigm, like

tabs, can be useful.

We can use tab navigation to allow the user to navigate to a number of different screens at the root

level. Tabs are suitable when a number of screens carry roughly equal importance.

Bottom navigation - (from Material Design documentation - Components)

https://reactnavigation.org/docs/stack-navigator.html#stacknavigatorconfig

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Favorites

Before we begin adding tab navigation components to our application, we’ll need to build a few more

screens. Let’s build the Favorites screen of our application first. We can create a Favorites.js file

within the screens directory and begin with its imports:

contact-list/5/screens/Favorites.js

import React from 'react';

import {

StyleSheet,

Text,

View,

FlatList,

ActivityIndicator,

} from 'react-native';

import { fetchContacts } from '../utils/api';

import ContactThumbnail from '../components/ContactThumbnail';

As we briefly described earlier in this chapter, this screen will be responsible for showing a list of

favorited contacts.

The Favorites component will not be a child of the Contacts component. And because we’ll be using

tab navigation as opposed to stack navigation, we can’t pass contacts to favorites via a navigation

prop.

Therefore, we’ll use the fetchContacts() function to fetch this data directly from the API. We

usually want to avoid making numerous API calls to obtain the same data on every screen. Once

we include a state container later in the chapter, we’ll remove this.

Note that we’ve set up the fetchContacts API call to randomly “favorite” contacts by setting the

favorites boolean on the contact object to true. That way, we should always have some contacts

displayed on this screen.

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contact-list/5/screens/Favorites.js

export default class Favorites extends React.Component {

static navigationOptions = {

title: 'Favorites',

};

state = {

contacts: [],

loading: true,

error: false,

};

async componentDidMount() {

try {

const contacts = await fetchContacts();

this.setState({

contacts,

loading: false,

error: false,

});

} catch (e) {

this.setState({

loading: false,

error: true,

});

}

The screen’s render function:

contact-list/5/screens/Favorites.js

renderFavoriteThumbnail = ({ item }) => {

const { navigation: { navigate } } = this.props;

const { avatar } = item;

return (

<ContactThumbnail

avatar={avatar}

onPress={() => navigate('Profile', { contact: item })}

);

Navigation 347

};

render() {

const { loading, contacts, error } = this.state;

const favorites = contacts.filter(contact => contact.favorite);

return (

{loading && <ActivityIndicator size="large" />}

{error && <Text>Error...</Text>}

{!loading &&

!error && (

<FlatList

data={favorites}

keyExtractor={keyExtractor}

numColumns={3}

contentContainerStyle={styles.list}

renderItem={this.renderFavoriteThumbnail}

)}

</View>

);

}

In render, we filter our list of contacts using a favorites flag. Using the same pattern we used in

Contacts, we show a loading indicator while the request is still being made, an error message if the

request fails, or the list of contacts.

We’re making use of the numColumns prop for FlatList to render three contacts in every row.

The renderFavoriteThumbnail method is responsible for every item in the list where we use our

ContactThumbnail component to display the user’s avatar. Notice how we’ve also passed in a

navigate action to the onPress prop. This allows the user to navigate to the contact’s profile screen

by pressing an avatar on the Favorites screen - just like when they press a contact on the Contacts

screen.

Since we’re using FlatList again, we can hook up a keyExtractor method once more:

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contact-list/5/screens/Favorites.js

import ContactThumbnail from '../components/ContactThumbnail';

const keyExtractor = ({ phone }) => phone;

export default class Favorites extends React.Component {

And we can finish things off here by adding a few styles:

contact-list/5/screens/Favorites.js

const styles = StyleSheet.create({

container: {

backgroundColor: 'white',

justifyContent: 'center',

flex: 1,

list: {

alignItems: 'center',

});

Try it out

In a moment, we’ll build the Users screen then add tab navigation to our app. Before we do, let’s

add Favorites to our stack navigator just so we can see what it looks like. We’ll set it as our initial

route:

contact-list/routes.js

import { StackNavigator } from 'react-navigation';

import Contacts from './screens/Contacts';

import Profile from './screens/Profile';

import Favorites from './screens/Favorites';

export default StackNavigator(

{

Contacts: {

screen: Contacts,

Profile: {

screen: Profile,

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Favorites: {

screen: Favorites,

{

initialRouteName: 'Favorites',

);

Favorites

Note that pressing on any avatar navigates to the Profile screen.

As Favorites will not be a part of our stack navigation, go ahead and revert the changes here to the

original route configurations.

User screen

Let’s now build the third root screen, User, which displays the details of the user of the app. We’ll

begin with its imports:

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contact-list/5/screens/User.js

import React from 'react';

import { StyleSheet, Text, View, ActivityIndicator } from 'react-native';

import ContactThumbnail from '../components/ContactThumbnail';

import colors from '../utils/colors';

import { fetchUserContact } from '../utils/api';

We’re using another method, fetchUserContact, from our API utility file. This fetches a single

contact.

We define the header styles at the top of the component:

contact-list/5/screens/User.js

export default class User extends React.Component {

static navigationOptions = {

title: 'Me',

headerTintColor: 'white',

headerStyle: {

backgroundColor: colors.blue,

};

Similar to the Profile screen, we’re displaying a blue header bar with white text.

Our local state and componentDidMount method will follow the same pattern as other screens:

contact-list/5/screens/User.js

state = {

user: [],

loading: true,

error: false,

};

async componentDidMount() {

try {

const user = await fetchUserContact();

this.setState({

user,

loading: false,

error: false,

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});

} catch (e) {

this.setState({

loading: false,

error: true,

});

}

Our render method will show the ContactThumbnail with the user’s name, avatar and phone

number:

contact-list/5/screens/User.js

render() {

const { loading, user, error } = this.state;

const { avatar, name, phone } = user;

return (

{loading && <ActivityIndicator size="large" />}

{error && <Text>Error...</Text>}

{!loading && (

)}

</View>

);

}

And finally, we’ll style our container to have a blue background as well as place our thumbnail in

the center of our screen:

contact-list/5/screens/User.js

const styles = StyleSheet.create({

container: {

flex: 1,

alignItems: 'center',

justifyContent: 'center',

backgroundColor: colors.blue,

});

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Try it out

We’ll again temporarily modify our routes.js file to test out this component:

contact-list/routes.js

import { StackNavigator } from 'react-navigation';

import Contacts from './screens/Contacts';

import Profile from './screens/Profile';

import User from './screens/User';

export default StackNavigator(

{

Contacts: {

screen: Contacts,

Profile: {

screen: Profile,

User: {

screen: User,

{

initialRouteName: 'User',

);

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User

Nested navigators

Now that we have all the screens that make up our tabs, we can start putting our tab navigation

logic in place. Let’s import and use the TabNavigator component in our routes.js file:

contact-list/routes.js

import { TabNavigator } from 'react-navigation';

import Contacts from './screens/Contacts';

import Favorites from './screens/Favorites';

import User from './screens/User';

export default TabNavigator({

Contacts: {

screen: Contacts,

Favorites: {

screen: Favorites,

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User: {

screen: User,

});

If we run the app, we’ll see tabs with labels at the bottom of the iOS device screen and at the top of

the Android screen. These tabs allow us to switch between the three screens.

Tabs

We’ll modify the styling of our tabs in a little bit. At the moment, we have a bigger problem. We

don’t have our header component anymore and if we try pressing any of the contacts in the Contacts

or Favorites screens, nothing happens. This is because we’ve removed our StackNavigator entirely

and only have a single tab navigation system in place. What we want to do is compose our two

navigators.

Let’s update our route configurations. We’ll begin with the imports since we’ll be including a few

new ones:

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contact-list/5/routes.js

import React from 'react';

import { StackNavigator, TabNavigator } from 'react-navigation';

import { MaterialIcons } from '@expo/vector-icons';

import Favorites from './screens/Favorites';

import Contacts from './screens/Contacts';

import Profile from './screens/Profile';

import User from './screens/User';

import colors from './utils/colors';

We’re importing both navigators from react-navigation as well as MaterialIcons from Expo’s

vector-icons package. This package is a wrapper around react-native-vector-icons

, a library

that contains a number of vector icons. Creating icons is done by using JSX to define icon

components. For this reason, we’ve imported React to this file as well.

You can find a list of all the icons provided by the library here

. There are a number

of different icon sets that can be used (such as FontAwesome). We’ll only be using the

MaterialIcons icon set for this application.

To compose stack navigation and tab navigation, each tab will have its own separate navigation

stack. This means that instead of passing specific screens to each tab, we’ll pass in a stack (a

StackNavigator component) that contains every possible screen within that tab.

For example, we want the contacts list to still use a stack navigator. That way, the user can navigate

to individual profiles. That stack navigator will reside inside our app’s broader tab navigator.

Let’s write our stack navigator for the contacts list. It looks the same as before, with one additional

configuration:

contact-list/5/routes.js

const ContactsScreens = StackNavigator(

{

Contacts: {

screen: Contacts,

Profile: {

screen: Profile,

https://github.com/oblador/react-native-vector-icons

https://expo.github.io/vector-icons/

Navigation 356

{

initialRouteName: 'Contacts',

navigationOptions: {

tabBarIcon: getTabBarIcon('list'),

);

For the “Contacts” tab, we want to first show the Contacts screen and allow the user to be able to

navigate to the Profile screen.

Notice how we’ve also added navigation options to specify this navigator’s tabBarIcon. We’re

passing in a getTabIcon helper method to retrieve a list icon component. Let’s define this function

right after our imports:

contact-list/5/routes.js

import colors from './utils/colors';

const getTabBarIcon = icon => ({ tintColor }) => (

);

const ContactsScreens = StackNavigator(

The tabBarIcon option expects a function. It will call that function with a single object that has the

properties focused and tintColor.

The getTabIcon function returns a function that returns a specific icon component from the

MaterialIcons icon set given its name. When we define our TabNavigator component, we’ll assign

the tint colors for all icons in each of our tabs.

Higher-order functions

The tabBarIcon parameter in our navigation options expects a function that takes an object with

focused and tintColor as attributes. It looks like the following:

tabBarIcon: (params) => (

<MaterialIcons

name="list"

size={26}

style={{ color: params.tintColor }}

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The focused argument allows us to render something different depending on whether the current

tab is focused in view or not. We’re not doing anything for the different states so we do not even

use focused at all. We only use tintColor in order to give our tab icons an appropriate active or

inactive color defined as options in TabNavigator.

With parameter context matching

, we can simplify how we pass in our arguments:

tabBarIcon: ({ tintColor }) => (

<MaterialIcons

name="list"

size={26}

style={{ color: tintColor }}

Since every icon in each of the tabs have the same tint color and size, we simplify how we render

our icons by defining a single getTabIcon function:

const getTabBarIcon = icon => ({ tintColor }) => (

);

// ...

tabBarIcon: getTabBarIcon('list'),

The getTabIcon function takes an icon string as a parameter and returns a function that takes the

correct parameters expected by tabBarIcon (an object with tintColor and focused).

appendix_higher_order_components

Let’s do the same for our other two tabs, Favorites and User:

contact-list/5/routes.js

const FavoritesScreens = StackNavigator(

{

Favorites: {

screen: Favorites,

Profile: {

screen: Profile,

{

initialRouteName: 'Favorites',

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navigationOptions: {

tabBarIcon: getTabBarIcon('star'),

);

const UserScreens = StackNavigator(

{

User: {

screen: User,

{

initialRouteName: 'User',

navigationOptions: {

tabBarIcon: getTabBarIcon('person'),

);

For the Favorites tab, the idea is similar. We want to default to the Favorites screen but still allow

the user to navigate to the Profile screen for any specific contact. The User stack navigator only

has one screen at the moment, but we’ll add a second screen a little later.

Now that we’ve defined our stack navigators, we can write up our tab navigator underneath:

contact-list/5/routes.js

export default TabNavigator(

{

Contacts: {

screen: ContactsScreens,

Favorites: {

screen: FavoritesScreens,

User: {

screen: UserScreens,

{

initialRouteName: 'Contacts',

tabBarPosition: 'bottom',

tabBarOptions: {

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style: {

backgroundColor: colors.greyLight,

showLabel: false,

showIcon: true,

activeTintColor: colors.blue,

inactiveTintColor: colors.greyDark,

renderIndicator: () => null,

);

We pass each StackNavigator as the screen for their corresponding tab. React Navigation allows us

to pass entire navigators as a tab screen and the first screen of the stack will be the default screen

for that tab. We’ve also defined some configurations for our tabs:

• tabBarPosition allows us to have our tabs at the top or bottom of our screen. For iOS, this

defaults to bottom and Android defaults to top. Specifying bottom means the tabs will render

at the bottom for both platforms.

• tabBarOptions allow us to modify styling for our tabs. We first define the tab background color

using the style object. Since we only want icons to show, we’ve also specified showLabel to be

false and showIcon to true. We set our icon colors for both active (where the user is currently

viewing) and inactive tabs. The last option, renderIndicator, allows us to pass in a function

to modify how tab indicators (or lines at the bottom of the active tab) are rendered. A default

tab indicator shows for Android and we pass null to remove it entirely.

With regards to tabBarOptions, there are options specific to Android that do not apply

to iOS. Every option we’ve specified so far is customizable for both platforms. For more

information on tab configurations and options, refer to the documentation

Try it out

If we try running the application now, we’ll see our complete tab logic working!

https://reactnavigation.org/docs/navigators/tab

Navigation 360

Contacts

Try navigating between the different tabs as well as pressing a contact list item or thumbnail to

navigate to the Profile screen. You’ll notice that navigating to the Profile screen in one tab will

only show that screen there. Composing both tab and stack navigation allows for more complex

navigation architectures where each stack in a tab maintains the history of its own navigated screens

independently from the others.

Modal navigation

We’ll introduce our final screen in the app, Options, to demonstrate how stack navigators can be

modified to render new screens with a modal. This is only for iOS and does not work for Android.

We can begin by creating an Options.js file in the screens/ directory:

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contact-list/screens/Options.js

1 import React from 'react';

2 import { StyleSheet, View } from 'react-native';

3 import { MaterialIcons } from '@expo/vector-icons';

5 import DetailListItem from '../components/DetailListItem';

6 import colors from '../utils/colors';

8 export default class Options extends React.Component {

9 static navigationOptions = ({ navigation: { goBack } }) => ({

10 title: 'Options',

11 headerLeft: (

12 <MaterialIcons

13 name="close"

14 size={24}

15 style={{ color: colors.black, marginLeft: 10 }}

16 onPress={() => goBack()}

17 />

18 ),

19 });

21 render() {

22 return (

23 <View style={styles.container}>

24 <DetailListItem title="Update Profile" />

25 <DetailListItem title="Change Language" />

26 <DetailListItem title="Sign Out" />

27 </View>

28 );

29 }

30 }

32 const styles = StyleSheet.create({

33 container: {

34 flex: 1,

35 backgroundColor: 'white',

36 },

37 });

In this screen, we render a few DetailListItem components to represent options that the user

can press to modify their profile settings. We’ve included a headerLeft attribute for the screen’s

Navigation 362

navigation options that renders a close icon. We’ve added a callback to the icon’s onPress prop that

fires navigate.goBack to close the current screen and return to the previous one.

Let’s build the functionality to allow the user to navigate to this screen. We’ll do this in the User

screen by adding an icon to our header:

contact-list/screens/User.js

static navigationOptions = ({ navigation: { navigate } }) => ({

title: 'Me',

headerTintColor: 'white',

headerStyle: {

backgroundColor: colors.blue,

headerRight: (

<MaterialIcons

name="settings"

size={24}

style={{ color: 'white', marginRight: 10 }}

onPress={() => navigate('Options')}

});

Don’t forget to import MaterialIcons in this file.

Now, in routes.js, we’ll import our Options screen:

contact-list/routes.js

import Options from './screens/Options';

Then add it to the UserScreens stack navigator within routes.js:

contact-list/routes.js

const UserScreens = StackNavigator(

{

User: {

screen: User,

Options: {

screen: Options,

{

Navigation 363

mode: 'modal',

initialRouteName: 'User',

navigationOptions: {

tabBarIcon: getTabBarIcon('person'),

);

We use the mode attribute to specify this navigator should have modal transitions for iOS.

Try it out

Start the app and give it a shot! You’ll be able to navigate directly to the Options screen through

User.

User

By pressing the icon on the right of our header bar, you’ll notice that the screen moves up from the

bottom if you own an iOS device.

Navigation 364

Options

If you try on Android device or emulator, the screen will fade in just like any of the other screens

in the stack.

Drawer navigation

Another navigation pattern that is commonly used is drawer navigation, where views are accessible

through a drawer that slides in from the left side of the screen.

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Navigation drawer - (from Material Design documentation - Components)

Just like tab navigation, a drawer navigator allows users to switch between equally important views

quickly.

Neither of these patterns is better than the other. Choosing the right pattern depends on both

preferences as well as the number of root screens that the user can access.

A good rule of thumb is that tabs work well when there are three to five of them. If there are many

important, unrelated screens that the user should be able to access without navigating through a

stack, then drawer navigation may be more suitable.

Although our final app only includes tab navigation for our three core views, let’s explore what

using drawer navigation would look like. We’ll swap in DrawerNavigator for TabNavigator.

Let’s modify our routes.js file:

contact-list/6/routes.js

1 import React from 'react';

2 import { StackNavigator, DrawerNavigator } from 'react-navigation';

3 import { MaterialIcons } from '@expo/vector-icons';

5 import Favorites from './screens/Favorites';

6 import Contacts from './screens/Contacts';

7 import Profile from './screens/Profile';

8 import User from './screens/User';

9 import Options from './screens/Options';

11 const getDrawerItemIcon = icon => ({ tintColor }) => (

Navigation 366

12 <MaterialIcons name={icon} size={22} style={{ color: tintColor }} />

13 );

15 const ContactsScreens = StackNavigator(

16 {

17 Contacts: {

18 screen: Contacts,

19 },

20 Profile: {

21 screen: Profile,

22 },

23 },

24 {

25 initialRouteName: 'Contacts',

26 navigationOptions: {

27 drawerIcon: getDrawerItemIcon('list'),

28 },

29 },

30 );

32 const FavoritesScreens = StackNavigator(

33 {

34 Favorites: {

35 screen: Favorites,

36 },

37 Profile: {

38 screen: Profile,

39 },

40 },

41 {

42 initialRouteName: 'Favorites',

43 navigationOptions: {

44 drawerIcon: getDrawerItemIcon('star'),

45 },

46 },

47 );

49 const UserScreens = StackNavigator(

50 {

51 User: {

52 screen: User,

53 },

54 Options: {

Navigation 367

55 screen: Options,

56 },

57 },

58 {

59 mode: 'modal',

60 initialRouteName: 'User',

61 navigationOptions: {

62 drawerIcon: getDrawerItemIcon('person'),

63 },

64 },

65 );

67 export default DrawerNavigator(

68 {

69 Contacts: {

70 screen: ContactsScreens,

71 },

72 Favorites: {

73 screen: FavoritesScreens,

74 },

75 User: {

76 screen: UserScreens,

77 },

78 },

79 {

80 initialRouteName: 'Contacts',

81 },

82 );

Note that instead of the tabBarIcon option, we use drawerIcon to display an icon near the menu item

within the drawer. We also change the name of the method that returns our icon from getTabIcon

to getDrawerIcon.

Although the drawer can be accessed by swiping on the left edge of the device screen towards the

right, it also helps to have a menu icon in each of the main screens that can open and close the

drawer. We’ll begin with the Contacts screen:

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contact-list/6/screens/Contacts.js

static navigationOptions = ({ navigation: { navigate } }) => ({

title: 'Contacts',

headerLeft: (

<MaterialIcons

name="menu"

size={24}

style={{ color: colors.black, marginLeft: 10 }}

onPress={() => navigate('DrawerToggle')}

});

We’ve added a menu icon on the left hand side of the header. With React Navigation, we can navigate

to DrawerOpen and DrawerClose to open and close the drawer respectively. DrawerToggle will fire

either of those depending on the current state of the navigation drawer. This allows us to toggle the

drawer with a single method.

We can add this same icon to the Favorites screen:

contact-list/6/screens/Favorites.js

static navigationOptions = ({ navigation: { navigate } }) => ({

title: 'Favorites',

headerLeft: (

<MaterialIcons

name="menu"

size={24}

style={{ color: colors.black, marginLeft: 10 }}

onPress={() => navigate('DrawerToggle')}

});

And the User screen:

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contact-list/6/screens/User.js

static navigationOptions = ({ navigation: { navigate } }) => ({

title: 'Me',

headerTintColor: 'white',

headerStyle: {

backgroundColor: colors.blue,

headerLeft: (

<MaterialIcons

name="menu"

size={24}

style={{ color: 'white', marginLeft: 10 }}

onPress={() => navigate('DrawerToggle')}

headerRight: (

<MaterialIcons

name="settings"

size={24}

style={{ color: 'white', marginRight: 10 }}

onPress={() => navigate('Options')}

});

Try it out

Try running the application with these drawer settings enabled. We can see a menu icon in either

of our three root screens.

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Contacts Screen - Drawer

We can open and close our drawer by swiping right on the left edge of the screen or by pressing the

menu icon.

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Drawer

Sharing state between screens

So far, we’ve built our entire application using local component state. While doing so, we noticed

some of the challenges that applications with multiple screens have when data must be shared

between screens.

For example, our Favorites screen uses the same list of contacts as the Contacts screen. But we had

to make an API call for each screen, fetching the list twice. It would be better to fetch the list just

once and share data between screens.

There are a few different ways we can make this better. One way is to define all the data within

our application in the App component. We can then use the initialRouteParams property that React

Navigation provides for our root tab navigator to assign the state to its initial route - Contacts.

With this approach, we can continue to pass our entire data to every other screen we navigate using

navigation parameters similar to how we pass contact information from Contacts to Profile.

This method of maintaining state is not ideal due to the fact that every screen now has access to all

the data in the entire app. Moreover, this will most likely create performance issues as we would

need to re-render every screen to reflect changes to the state being modified in a specific screen.

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State containers

React Native applications that contain a navigation architecture generally handle data flow differ-

ently than we’ve done in our apps so far. So far, we’ve stored data in the root and screen components

of our apps, and we’ve passed data down from parent to child as props. In this app, we’ll use a

state container to manage all of our application data in a separate external location outside of our

components.

This can be useful to separate the UI and data concerns in our application. In a typical application

with multiple top-level screens, this approach allows us to pass parts of our state to each of our

screens.

Third-party libraries

One approach to including a state container is to use a third-party library. Redux

and MobX

are

two popular examples that allow users to maintain their entire application state in a single location.

They also impose certain restrictions on how this state object can be modified.

Using a community-supported library means we don’t have to spend the time trying to build the

logic ourselves. There are packages that exist in both the Redux and MobX ecosystem that allow

us to bind our React or React Native components directly to the global store. Modifying our state

requires an action to be dispatched which gives us more explicit control to modify our state in only

specific parts of our application. We can also take advantage of middlewares to intercept any of

our actions before it reaches our state. This can allow us to log all of our state changes for easier

debugging as well as fire asynchronous operations as part of our actions if we need to.

Downsides of using an external library include the learning curve needed to learn its API and specific

requirements. Redux, for example, adheres to the use of pure functions (or functions without any

side effects) and requires a decent amount of boilerplate code in order to connect a component to

our store with appropriate actions. MobX uses the concept of reactive programming and observables

in order to have our state update when our data is changed. Using either of these libraries or another

state management tool altogether means that we would need to fully understand how they work.

Custom state container

Instead of using a community-supported library, we always have the option of building our own

state container in our application. This can be useful when we want complete control over how we

manage our data, or when we don’t want to introduce a complex dependency.

While suitable for our needs in this app, a simple state container built from scratch will not have

nearly as many features or plugins as a third-party library. Most medium and large React Native

applications use Redux or MobX. These libraries have a bit of a learning curve and require more

boilerplate code, but they make things more predictable by constraining how we manage our state.

https://github.com/reactjs/redux

https://github.com/mobxjs/mobx

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They can help us build consistent applications that are easier to test, debug and analyze using

different built-in or external plugins.

To demonstrate the overall approach of how to manage state in a central location with navigation,

we’ll use an extremely simple state container of our own instead of relying on a third-party solution.

Copy over the the contact-list/store.js file in the sample code to the root of you application. If

you like, take a look at the file to get a better understanding of how it works.

In this file, we’ve defined all of our application state as a single object called state:

contact-list/store.js

1 let state = {

2 isFetchingContacts: true,

3 isFetchingUser: true,

4 contacts: [],

5 user: {},

6 error: false,

7 };

Each of the field values in this object are the default values when our application is first launched.

We then export three methods that can be used throughout our application:

• getState returns the application state.

• setState takes in new values and updates our state. We don’t mutate the current state object

directly, but instead create a new copy with our updated values.

• onChange allows us to listen for changes in our state.

Let’s begin by including it to our Contacts screen. We’ll start with importing our store:

contact-list/7/screens/Contacts.js

import store from '../store';

Now we can make use of our store methods:

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contact-list/7/screens/Contacts.js

export default class Contacts extends React.Component {

static navigationOptions = {

title: 'Contacts',

};

state = {

contacts: store.getState().contacts,

loading: store.getState().isFetchingContacts,

error: store.getState().error,

};

async componentDidMount() {

this.unsubscribe = store.onChange(() =>

this.setState({

contacts: store.getState().contacts,

loading: store.getState().isFetchingContacts,

error: store.getState().error,

}));

const contacts = await fetchContacts();

store.setState({ contacts, isFetchingContacts: false });

}

componentWillUnmount() {

this.unsubscribe();

}

We’ve set up our local state by connecting its attributes to the correct global store attributes. Notice

how we’ve defined loading to be equal to the global state attribute of isFetchingContacts. This

component does not need to know anything else in the state that it is not using (and this includes

the isFetchingUser boolean that would be used in the User screen). We have complete control of

how we want to refer and define our state relevant to the context of this component.

In componentDidMount, we set our onChange method to update our local component state using

this.setState. When we get the results of our API call, we use our store setState method to

update our shared store and since we also use onChange - our local component state will also update

to reflect this change. We make sure to unsubscribe from our change listener when our component

unmounts as well.

Now in our render method, we’ll need to update which parameters we look for within our state:

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contact-list/7/screens/Contacts.js

render() {

const { contacts, loading, error } = this.state;

const contactsSorted = contacts.sort((a, b) =>

a.name.localeCompare(b.name));

return (

{loading && <ActivityIndicator size="large" />}

{error && <Text>Error...</Text>}

{!loading &&

!error && (

<FlatList

data={contactsSorted}

keyExtractor={keyExtractor}

renderItem={this.renderContact}

)}

</View>

);

}

Let’s do the same thing for Favorites:

contact-list/7/screens/Favorites.js

export default class Favorites extends React.Component {

static navigationOptions = {

title: 'Favorites',

};

state = {

contacts: store.getState().contacts,

loading: store.getState().isFetchingContacts,

error: store.getState().error,

};

async componentDidMount() {

const { contacts } = this.state;

this.unsubscribe = store.onChange(() =>

this.setState({

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contacts: store.getState().contacts,

loading: store.getState().isFetchingContacts,

error: store.getState().error,

}));

if (contacts.length === 0) {

const fetchedContacts = await fetchContacts();

store.setState({ contacts: fetchedContacts, isFetchingContacts: false });

}

componentWillUnmount() {

this.unsubscribe();

}

We technically shouldn’t need to submit fetch our contacts from the API again. We know that when

the user loads the application for the first time, the contacts are retrieved within the Contacts screen,

which is the first tab. However, it’s generally better not rely on the order each screen loads to ensure

our data is ready. This is both for testability (we can easily run this screen individually), and because

if we later add the capability to navigate to this screen without loading the Contacts screen first,

our application will fail. A good example of when this might happen is when we allow a user to

deep link to a specific screen from outside the application entirely. We’ll explore this concept in a

bit.

Similarly, we can update our render method as well:

contact-list/7/screens/Favorites.js

render() {

const { contacts, loading, error } = this.state;

const favorites = contacts.filter(contact => contact.favorite);

return (

{loading && <ActivityIndicator size="large" />}

{error && <Text>Error...</Text>}

{!loading &&

!error && (

<FlatList

data={favorites}

keyExtractor={keyExtractor}

numColumns={3}

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contentContainerStyle={styles.list}

renderItem={this.renderFavoriteThumbnail}

)}

</View>

);

}

And finally, we can update our User screen to follow this same approach:

contact-list/7/screens/User.js

export default class User extends React.Component {

static navigationOptions = {

title: 'Me',

headerTintColor: 'white',

headerStyle: {

backgroundColor: colors.blue,

};

state = {

user: store.getState().user,

loading: store.getState().isFetchingUser,

error: store.getState().error,

};

async componentDidMount() {

this.unsubscribe = store.onChange(() =>

this.setState({

user: store.getState().user,

loading: store.getState().isFetchingUser,

error: store.getState().error,

}));

const user = await fetchUserContact();

store.setState({ user, isFetchingUser: false });

}

componentWillUnmount() {

this.unsubscribe();

}

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render() {

const { user, loading, error } = this.state;

const { avatar, name, phone } = user;

return (

{loading && <ActivityIndicator size="large" />}

{error && <Text>Error...</Text>}

{!loading && (

)}

</View>

);

}

Don’t forget to import store within Favorites and User!

Try it out

If we try running the application at this point, we’ll notice everything works exactly the same. Again,

the difference now is that each top level screen in our application uses the same shared data object

in our application.

Instead of being more specific about which parts of our central store we wanted to connect to in

each screen, we could have just connected the entire store using state = store.getState();.

However, connecting only parts of the global state to our component not only improves how we

encapsulate which state parameters we need, but it can also improve re-render performance. In

React Native, a component re-renders if any part of its state changes. With this approach, we can

have our component re-render only when the state specific to it changes.

Although a centralized store allowed us to handle how data is managed across a number of top-level

screens, it’s important to remember that this adds an extra layer of abstraction to our application.

Not only do we now pass presentational data from parent components to child components, we

also pass data through navigation parameters when we navigate through certain screens and use a

top-level state container that manages all the data in our application.

Deep Linking

The last major topic we’ll explore in this chapter is deep linking. Deep linking means launching

the app and navigating to a specific screen automatically. A deep link bypasses the tab, stack, and

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drawer navigation, taking the user directly to the desired screen. This can be useful when launching

your app from a webpage, a push notification, or another app.

Imagine the user gets a push notification that a new contact has been added. When the user taps the

notification, they should be taken directly to the profile screen for that contact, rather than having

to navigate their way from the initial screen of the app.

Deep links are similar to typing a URI (Uniform Resource Identifier) into a web browser. On the web,

https://www.fullstackreact.com might load the homepage for the website, while https://www.fullstackreact.com/react-native

will load a different page. Similarly in mobile applications, we perform a deep link using a URI, where

each URI generally takes us to a different screen.

Using a navigation library for a mobile app helps us build our app in terms of screens – this makes

it easy to connect any screen of our app to a specific URI.

Let’s explore how we can add deep linking to our current application by allowing the user to navigate

to a specific contact’s profile directly. With Expo, the base URI is different based on the state of the

application:

• exp://localhost:19000/+ or exp://10.2.8.358:19000/+ is the URI we use during develop-

ment. 10.2.8.358 is our IP address and 19000 is the port that the app is running. We can see

our IP address and the port right underneath the QR code printed to the terminal when we

start the application with yarn start.

• exp://exp.host/@fullstackio/contact-list/+ is what we can use if our app is published to

the Expo client. If you have the final version of this app installed on your device through the

client, try typing this address into a web browser on your mobile device and you’ll navigate

directly to it.

• For standalone apps published outside of Expo, the URI can be defined in app.json. For

example:

app.json

{

"expo": {

"scheme": "contact-list"

}

This will give us a URI of contact-list://+.

Instead of having to take care of all of these different possible URI values, Expo provides us with a

linkingUri attribute from a Constants object that will resolve to the correct URI depending on the

state of the application.

Now let’s begin adding deep linking to our contact list application! Our goal is to allow a user to navi-

gate to a specific contact only using his or her first name. For example, exp://{linkingUri}/+?name=ali

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will navigate directly to Ali’s profile. If the contact doesn’t exist, we’ll have the user remain in the

Contacts screen and not be navigated anywhere. For a real production application however, it would

make more sense to show a user-friendly error message if this happens.

We’ll begin with importing Linking and Constants into our Contacts screen:

contact-list/screens/Contacts.js

import React from 'react';

import {

StyleSheet,

Text,

View,

FlatList,

ActivityIndicator,

Linking,

} from 'react-native';

The Linking API provides methods that allow us to handle incoming deep links as well as open

external links. We’ll add two of these methods to the lifecycle hook that fires after our component

mounts:

contact-list/screens/Contacts.js

async componentDidMount() {

this.unsubscribe = store.onChange(() =>

this.setState({

contacts: store.getState().contacts,

loading: store.getState().isFetchingContacts,

error: store.getState().error,

}));

const contacts = await fetchContacts();

store.setState({ contacts, isFetchingContacts: false });

Linking.addEventListener('url', this.handleOpenUrl);

const url = await Linking.getInitialURL();

this.handleOpenUrl({ url });

}

Let’s go over the two methods we added:

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• The getInitialURL method will fire when a URI associated with the app is accessed externally.

This method allows a user to deep link to a particular part of the application when the

app is closed and not running in the background. In here, we pass the URL obtained to a

handleOpenUrl method.

• For instances where the app is running in the background, we can listen to URL events and

provide a callback to handle these situations. This is why we use addEventListener and pass

a handler to the same handleOpenUrl method.

Like any event listener, we’ll need to make sure it is removed when our component is destroyed/un-

mounted:

contact-list/screens/Contacts.js

componentWillUnmount() {

Linking.removeEventListener('url', this.handleOpenUrl);

this.unsubscribe();

}

Now that we’ve handled incoming deep links in our component, let’s create our handler method to

respond appropriately to the correct URL:

contact-list/screens/Contacts.js

handleOpenUrl(event) {

const { navigation: { navigate } } = this.props;

const { url } = event;

const params = getURLParams(url);

if (params.name) {

const queriedContact = store

.getState()

.contacts.find(contact =>

contact.name.split(' ')[0].toLowerCase() ===

params.name.toLowerCase());

if (queriedContact) {

navigate('Profile', { id: queriedContact.id });

}

We use a getURLParams utility function to extract query parameters from a given string and return

an object. For example getURLParams(exp://localhost:19000/+?name=abby) will return {name:

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'abby'}. If the name parameter exists, we check our state for a contact with the same first name

and if so - navigate to the Profile screen for that user.

We’ll also need to import the getURLParams utility function at the top of our file:

contact-list/screens/Contacts.js

import getURLParams from '../utils/getURLParams';

Although deep linking to a contact by name is straightforward, this approach is flawed. If two

contacts have the same name, our logic will fail, always returning the first contact it finds that

meets the condition. Ideally, we would want to pass the contact’s ID as a URI parameter. In this

example application however, we generate random UUIDs for each contact every time we load our

application. For this reason, we’ve shown the name-based approach for simplicity.

Try it out

While developing locally, you can try deep linking with different methods depending on which

platform you’re using:

• If you’re using the iOS simulator or an actual device connected to the same network, you can

open Safari and type exp://localhost:19000/+?name=ali into the address bar.

• On an Android emulator on the same network, you can test it through a terminal command:

adb shell am start -W -a "android.intent.action.VIEW" -d "exp://localhost:19000/+?na\

me=ali"

If localhost:19000 isn’t working, the application may be running on a different port. Take

a look at the terminal to see which port is being used.

You’ll notice you’ll be navigated directly to his profile screen:

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Deep Linking

If we try navigating to a contact with a name that doesn’t exist, we’ll remain in the Contacts screen.

Summary

Navigation is one of the most crucial elements of building an application that requires multiple

screens. In this chapter, we looked at different navigation patterns as well as how they can be

composed to allow for a complete navigation system. We built out a complete contact list example

app to explore this in-depth, using all of the navigators provided by React Navigation. We then

moved on to building a small state container to further understand how data can be shared between

top-level screens. Finally, we finished the chapter by including deep linking functionality in a specific

part of our application.

While discussing the differences between native and JavaScript navigation libraries, we briefly

touched on how JavaScript implementaions use their own custom components by relying on React

Native components and the Animated API. Animations and gestures are important topics in mobile

development and the next two chapters will dive deeper into how they work in React Native.

Animation

In order to animate a component on the screen, we’ll generally update its size, position, color, or

other style attributes continuously over time. We often use animations to imitate movement in the

real world, or to build interactivity similar to familiar physical objects.

We’ve seen several examples of animation already:

• In our weather app, we saw how the KeyboardAvoidingView shrinks to accommodate room for

the keyboard.

• In our messaging app, we used the LayoutAnimation API to achieve similar keyboard-related

animations.

• In our contacts app, we used react-nagivation, which automatically coordinates transition

animations between screens.

In this chapter and the next, we’ll explore animations in more depth by building a simple puzzle game

app. React Native offers two main animation APIs: Animated and LayoutAnimation. We’ll use these

to create a variety of different animations in our game. Along the way, we’ll learn the advantages

and disadvantages of each approach.

The next chapter (“Gestures”) will primarily focus on a closely related topic: gestures. Gestures help

us build components that respond to tapping, dragging, pinching, rotating, etc. Combining gestures

and animations enables us to build intuitive, interactive experiences in React Native.

Animation challenges

Building beautiful animations can be tricky. Let’s look at a few of the challenges we’ll face, and how

we can overcome them.

Performance challenges

To achieve animations that look smooth, we’ll want our UI to render at 60 frames-per-second (fps).

In other words, we need to render 1 frame roughly every 16 milliseconds (1000 milliseconds / 60

frames). If we perform expensive computations that take longer than 16 milliseconds within a single

frame, our animations may start to look choppy and uneven. Thus, we must constantly pay attention

to performance when working with animation.

Performance issues tend to fall into a few specific categories:

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• Calculating new layouts during animation: When we change a style attribute that affects

the size or position of a component, React Native usually re-calculates the entire layout of the

UI. This calculation happens on a native thread, but is still an expensive calculation that can

result in choppy animations. In this chapter, we’ll learn how we can avoid this by animating

the transform style attribute of a component.

• Re-rendering components: When a component’s state or props change, React must de-

termine how to reconcile these changes and update the UI to reflect them. React is fairly

efficient by default, so components generally render quickly enough that we don’t optimize

their performance. With animation, however, a large component that takes a few milliseconds

to render may lead to choppy animations. In this chapter, we’ll learn how we can reduce re-

renders with shouldComponentUpdate to acheive smoother animations.

• Communicating between native code and JavaScript: Since JavaScript runs asynchronously,

JavaScript code won’t start executing in response to a gesture until the frame after the gesture

happens on the native side. If React Native must pass values back and forth between the native

thread and the JavaScript engine, this can lead to slow animations. In this chapter, we’ll learn

how we can use useNativeDriver with our animations to mitigate this.

Complex control flow challenges

When working with animations, we tend to write more asynchronous code than normal. We must

often wait for an animation to complete before starting another animation (using an imperative API)

or unmounting a component. The asynchronous control flow and imperative calls can quickly lead

to confusing, buggy code.

To keep our code clear and accurate, we’ll use a state machine

approach for our more complex

components. We’ll define a set of named states for each component, and define transitions from

one state to another. This is similar to the React component lifecycle: our component will transition

through different states (similar to mounting, updating, unmounting, etc), and we can run a specific

function every time the state changes.

If you’re familiar with state machines, you might be wondering how our state machine

approach will differ from normal usage of React component state. React state is an implicit

state machine, which we often use without defining specific states. In our case, we’re going to

be explicit about our states, naming them and defining transitions between them. Ultimately

though, we’re just using React state in a slightly more structured way than normal.

If you’re not familiar with state machines, that’s fine. We’ll be building ours together as we

go through the chapter. Even if you’re not familiar with the term “state machine,” the coding

style will probably look familiar, since we’ve used it elsewhere in this book already (e.g. the

INPUT_METHOD from the “Core APIs” chapter).

https://en.wikipedia.org/wiki/Finite-state_machine

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Building a puzzle game

In this chapter, we’ll learn how to use the React Native animation APIs to build a slider-puzzle game.

You can try the completed app on your phone by scanning this QR code from within the Expo app:

Our app will have two screens. The first screen let’s us choose the size of the puzzle and start a new

game:

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The second screen opens when we start the game. The goal of the game is to rearrange the squares

of the puzzle to complete the image displayed in the top left:

Project setup

In this chapter, we’ll work out of the sample code directory for the project. Throughout this book

we’ve already set up several projects and created many components from scratch, so for this project

the import statements, propTypes, defaultProps, and styles are already written for you. We’ll be

adding the animations and interactivity to these components as we go.

We’ll use the contents of puzzle/1 as a foundation for our app. First, you’ll need to copy the

puzzle/1 directory to somewhere else on your computer. Then in the terminal, you can navigate to

the directory you copied and install the dependencies by running yarn.

For example, if you unzipped the book’s sample code to ∼/Downloads/fsrn/, on a macOS or Linux

computer you would run:

$ cp -r ~/Downloads/fsrn/puzzle/1 ~/Downloads/puzzle

$ cd ~/Downloads/puzzle

$ yarn

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You won’t be able to work out of the original directory, since it’s nested within another React

Native app directory, and the packager currently doesn’t support nested directories like this.

That’s why you’ll need to copy puzzle/1 elsewhere.

Once this finishes, choose one of the following to start the app:

• yarn start - Start the Packager and display a QR code to open the app on your phone

• yarn ios - Start the Packager and launch the app on the iOS simulator

• yarn android - Start the Packager and launch the app on the Android emulator

You should see a dark full-screen gradient (it’s subtle), which looks like this:

Project Structure

Let’s take a look at the files in the directory we copied:

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├── App.js

├── README.md

├── app.json

├── assets

│ ├── logo.png

│ ├── logo@2x.png

│ └── logo@3x.png

├── components

│ ├── Board.js

│ ├── Button.js

│ ├── Draggable.js

│ ├── Logo.js

│ ├── Preview.js

│ ├── Stats.js

│ └── Toggle.js

├── package.json

├── screens

│ ├── Game.js

│ └── Start.js

├── utils

│ ├── api.js

│ ├── clamp.js

│ ├── configureTransition.js

│ ├── controlFlow.js

│ ├── formatElapsedTime.js

│ ├── grid.js

│ ├── puzzle.js

│ └── sleep.js

├── validators

│ └── PuzzlePropType.js

└── yarn.lock

Here’s a quick overview of the most important parts:

• The App.js file is the entry point of our code, as with our other apps.

• The assets directory contains a logo for our puzzle app.

• The components directory contains all the component files we’ll use in this chapter. Some of

them have been written already, while others are scaffolds that need to be filled out.

• The screens directory contains the two screen components in our app: the Start screen and

the Game screen. The App coordinates the transitions between these two screens.

• The utils directory contains a variety of utility functions that let us build a complex app like

this more easily. Most of these functions aren’t specific to React Native, so you can think of

Animation 390

them as a “black box” – we’ll cover the relevant APIs, but the implementation details aren’t

too important to understand.

• The validators directory contains a custom propTypes function that we’ll use in several

different places.

Now that we’re familiar with the project structure, let’s dive into the code!

App

Let’s walk through how the App component coordinates different parts of the app. Open up App.js.

App state

App stores the state of the current game and renders either the Start screen or the Game screen. A

“game” in our app is represented by the state of the puzzle and the specific image used for the puzzle.

To start a new game, the app generates a new puzzle state and chooses a new random image.

If we look at the state object, we can see there are 3 fields:

puzzle/1/App.js

state = {

size: 3,

puzzle: null,

image: null,

};

• size - The size of the slider puzzle, as an integer. We’ll allow puzzles that are 3x3, 4x4, 5x5,

or 6x6. We’ll allow the user to choose a different size before starting a new game, and we’ll

initialize the new puzzle with the chosen size.

• puzzle - Before a game begins or after a game ends, this value is null. If there’s a current game,

this object stores the state of the game’s puzzle. The state of the puzzle should be considered

immutable. The file utils/puzzle.js includes utility functions for interacting with the puzzle

state object, e.g. moving squares on the board (which returns a new object).

• image - The image to use in the slider puzzle. We’ll fetch this image prior to starting the game

so that (hopefully) we can fully download it before the game starts. That way we can avoid

showing an ActivityIndicator and delaying the game.

App screens

Our app will contain two screens: Start.js and Game.js. Let’s briefly look at each.

Start screen

Open Start.js. The propTypes have been defined for you:

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puzzle/1/screens/Start.js

static propTypes = {

onChangeSize: PropTypes.func.isRequired,

onStartGame: PropTypes.func.isRequired,

size: PropTypes.number.isRequired,

};

When we write the rest of this component, we’ll be building the buttons that allow switching the

size of the puzzle board. We’ll receive the current size as a prop, and call onChangeSize when we

want to update the size in the state of App. We’ll also build a button for starting the game. When

the user presses this button, we’ll call the onStartGame prop so that App knows to instantiate a puzzle

object and transition to the Game screen.

The state object for this component includes a field transitionState:

puzzle/1/screens/Start.js

state = {

transitionState: State.Launching,

};

This transitionState value indicates the current state of our state machine. Each possible state is

defined in an object called State near the top of the file:

puzzle/1/screens/Start.js

const State = {

Launching: 'Launching',

WillTransitionIn: 'WillTransitionIn',

WillTransitionOut: 'WillTransitionOut',

};

This object defines the possible states in our state machine. We’ll set the component’s transitionState

to each of these values as we animate the different views in our component. We’ll then use

transitionState in the render method to determine how to render the component in its current

state.

We define the possible states as constants in State, rather than assigning strings directly

to transitionState, both to avoid small bugs due to typos and to clearly document all the

possible states in one place.

We can see that the Start screen begins in the Launching state, since transitionState is initialized

to State.Launching. The Start component will transition from Launching when the app starts, to

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WillTransitionIn when we’re ready to fade in the UI, to WillTransitionOut when we’re ready to

transition to the Game screen.

We’ll use this pattern of State and transitionState throughout the components in this app to keep

our asynchronous logic clear and explicit.

Game screen

Now open Game.js. Again, the propTypes have been defined for you:

puzzle/1/screens/Game.js

static propTypes = {

puzzle: PuzzlePropType.isRequired,

image: Image.propTypes.source,

onChange: PropTypes.func.isRequired,

onQuit: PropTypes.func.isRequired,

};

The puzzle and image props are used to display the puzzle board. When we want to change the

puzzle, we’ll pass an updated puzzle object to App using the onChange prop. We’ll also present a

button to allow quitting the game. When the user presses this button, we’ll call onQuit, initiating a

transition back to the Start screen.

Like in Start.js, we’ll use a state machine to simplify our code:

puzzle/1/screens/Game.js

const State = {

LoadingImage: 'LoadingImage',

WillTransitionIn: 'WillTransitionIn',

RequestTransitionOut: 'RequestTransitionOut',

WillTransitionOut: 'WillTransitionOut',

};

We’ll cover these states in more detail when we build the screen.

Now that you have an overview of how the state and screens of our app will work, we can dive in

to building animations!

Building the Start screen

In order to build the Start screen, we’ll use the two main building blocks of animation: LayoutAnimation

and Animated. Each of these come with their own strengths and weaknesses. With LayoutAnimation

we can easily transition our entire UI, while Animated gives us more precise control over individual

values we want to animate.

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Initial layout

Let’s use LayoutAnimation to animate the position of a logo from the center of the screen to the top

of the screen.

Initially we’ll show this:

Then we’ll animate the position of the logo to turn it into this:

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We’re rendering placeholder buttons for now. We’ll style the buttons and add some custom

animations soon.

Open up Start.js. You’ll notice the component’s import statements, propTypes, state, and styles

are already defined.

Let’s start by returning the Logo and a few other components from our render method. Add the

following to render:

puzzle/screens/Start.js

// ...

render() {

const { size, onChangeSize } = this.props;

const { transitionState } = this.state;

return (

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</View>

<View>

</View>

<View>

</View>

);

}

// ...

After saving Start.js, you should see the end state of our animation:

We touched on LayoutAnimation in the second half of the “Core APIs” chapter, but let’s revisit how

it works before we use it.

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LayoutAnimation

LayoutAnimation automatically animates the entire UI of our application from its current layout

to the next layout. Elements that change position or size will be translated or scaled. Elements

that are added or removed between layouts will be animated too, and we can choose what this

animation looks like. We call LayoutAnimation.create to define an animation configuration, and

then LayoutAnimation.configureNext to enqueue the animation to run the next time render is

called.

The LayoutAnimation.create API takes three parameters:

• duration - The duration of the animation

• easing - The curve of the animation. We choose from a predefined set of curves: spring, linear,

easeInEaseOut, easeIn, easeOut, keyboard.

• creationProp - The style to animate when a new element is added: opacity or scaleXY.

The main advantages of LayoutAnimation are:

• We can animate our entire UI with a single function call, rather than starting one or several

animations for each individual component.

• We can animate flexbox layout attributes like justifyContent and alignItems, so we don’t

have to specify movement in terms of coordinates.

• The API fits nicely with React’s declarative rendering pattern – we specify the start and

end states of our UI by returning components from render as we normally would, and

LayoutAnimation figures out the details of how to transition between these states.

The main disadvantages are:

• We have limited control over individual animations and individual components, since every

updated layout property of every component animates simultaneously.

• We can only animate layout attributes, so if we want to animate other style attributes like color

or opacity we’ll need to use Animated.

Note that we’ve already added the boilerplate code to enable LayoutAnimation on Android

at the top of App.js.

puzzle/screens/Start.js

if (

Platform.OS === 'android' &&

UIManager.setLayoutAnimationEnabledExperimental

) {

UIManager.setLayoutAnimationEnabledExperimental(true);

}

As we mentioned in the Core APIs chapter, this will only be necessary until the

LayoutAnimation implementation on Android matures.

Animation 397

Animating the logo

To animate our logo, we want to initially not render the components below it, then start rendering

them and enqueue an animation with LayoutAnimation.configureNext. Since the outer View

component rendered from our Start component has justifyContent: 'space-around' in its style,

adding more content below the logo will move the logo up.

Our component starts in the Launching state, so let’s update render to only render the View

components below the logo when we’re not in the Launching state:

puzzle/screens/Start.js

// ...

render() {

const { size, onChangeSize } = this.props;

const { transitionState } = this.state;

return (

</View>

{transitionState !== State.Launching && (

<View>

<Toggle

options={BOARD_SIZES}

value={size}

onChange={onChangeSize}

</View>

)}

{transitionState !== State.Launching && (

<View>

</View>

)}

</View>

);

}

// ...

When the app is in the Launching state, only the logo will appear. If we were to save Start.js now,

our app would render the logo in the center of the screen indefinitely.

Animation 398

When we shift the transitionState from Launching to WillTransitionIn, render() will return the

two View components below the logo. This will push the logo up towards the top of the screen. We

can use LayoutAnimation to animate the logo’s movement and the other two components’ entrances.

Let’s write a componentDidMount method and call LayoutAnimation.configureNext within it and set

our transitionState to WillTransitionIn. We’ll also add a little delay using await and our utility

function sleep so we see the logo in the initial state for a short period of time before starting the

animation.

Add the following to Start.js:

puzzle/screens/Start.js

// ...

async componentDidMount() {

await sleep(500);

const animation = LayoutAnimation.create(

750,

LayoutAnimation.Types.easeInEaseOut,

LayoutAnimation.Properties.opacity,

);

LayoutAnimation.configureNext(animation);

this.setState({ transitionState: State.WillTransitionIn })

}

// ...

After saving Start.js, you should see the animation! The logo should move up toward the top of

the screen, and the “Choose Size” text and placeholder buttons will fade in.

Awaiting LayoutAnimation

Suppose we want to change the timing of the animations so that the buttons don’t fade in until after

the logo finishing moving toward the top of the screen. To do this, we’ll need to know when our

LayoutAnimation completes.

We can use the second parameter of LayoutAnimation.configureNext(animation, completionCallback),

the completionCallback, to wait for an animation to finish. However, as of React Native 0.52, this

callback currently only works on iOS. To overcome this limitation, we’re going to use a utility

function that approximates the completion callback on Android (we’re going to assume that the

animation will complete in the duration we specified for the animation, e.g. 750 milliseconds).

Animation 399

Since we’re going to use the same animation in several different places throughout this app, the

utility function can also encapsulate some of the animation’s configuration parameters. We’ll also

use a promise-based API for our utility function for convenience.

The utility function is a little tricky, so we’ve already written it for you in configureTransition.js:

puzzle/utils/configureTransition.js

import { LayoutAnimation, Platform } from 'react-native';

export default function configureTransition(onConfigured = () => {}) {

const animation = LayoutAnimation.create(

750,

LayoutAnimation.Types.easeInEaseOut,

LayoutAnimation.Properties.opacity,

);

const promise = new Promise(resolve => {

// Workaround for missing LayoutAnimation callback support on Android

if (Platform.OS === 'android') {

LayoutAnimation.configureNext(animation);

setTimeout(resolve, 750);

} else {

LayoutAnimation.configureNext(animation, resolve);

}

});

onConfigured();

return promise;

}

If you don’t follow exactly what happens with the callback argument and promise, that’s

fine. The details of this aren’t important, and the need for this function should go away in a

subsequent React Native release.

We allow passing an onConfigured parameter so that we have a place to easily update component

state with setState between the time we call LayoutAnimation.create and the time we actually

schedule the animation with LayoutAnimation.configureNext.

If we call LayoutAnimation.configureNext before setState, most animations will still work

correctly, but there are a few instances where it won’t work on Android.

Let’s update componentDidMount in Start to use our configureTransition utility function:

Animation 400

puzzle/screens/Start.js

// ...

async componentDidMount() {

await sleep(500);

await configureTransition(() => {

this.setState({ transitionState: State.WillTransitionIn });

});

// ...

}

// ...

When you save the app now, you should see the exact same animation as before. The updated

code is a bit simpler and handles the completion callback, so that’s how we’ll use LayoutAnimation

throughout the rest of the app.

Animating buttons

Now that we can wait for the LayoutAnimation to complete, let’s update the animation of the buttons

to fade in after the logo finishes moving. We could use LayoutAnimation again, but instead we’re

going to learn how to make the same fade animation using the Animated API.

Animated

The Animated API lets us animate most style attributes of core components. While LayoutAnimation

is designed to automatically transition between two states, Animated lets us fine-tune the tim-

ing and duration of individual style attributes. Although Animated gives us more control than

LayoutAnimation, it’s also more complex: we must define which style attributes we want to animate

individually, and imperatively call functions to start these animations.

There are two parts to Animated:

• Animated.Value - This is a class that wraps a primitive value (number, string, etc) for use

in component styles. The Animated API includes functions for modifying the primitive value

within the wrapper, e.g. Animated.add.

• Animated.createAnimatedComponent(Component) - Components must be specially wrapped

with Animated.createAnimatedComponent in order to handle Animated.Value in their styles.

The Animated API exports a wrapped version of some of the most common components:

Animated.View, Animated.Text, Animated.Image, and Animated.ScrollView. These are just for

convenience – Animated.View is equivalent to Animated.createAnimatedComponent(View).

Animation 401

Running animations

It’s time to make the fade animation for our buttons! Let’s begin by instantiating two Animated.Value

instances. One will represent the opacity of the Toggle component, and the other will represent the

opacity of the start Button component. While we could animate both components with a single

Animated.Value, by instead instantiating one for each component, we can animate the components

independently. In our case, we’ll use an animation option called delay to stagger the animations

independently.

We’ll instantiate these animated values as instance properties of our Start component. We’ll add

them below where we instantiate the initial state:

puzzle/screens/Start.js

state = {

transitionState: State.Launching,

};

toggleOpacity = new Animated.Value(0);

buttonOpacity = new Animated.Value(0);

We initialize each Animated.Value with a primitive value of 0. For our animation, this primitive

value will represent that opacity of our components: 0 represents a fully transparent style and 1

represents a fully opaque style.

To perform a fade-in animation, we instruct the Animated.Value to change its primitive value from 0

to 1 over a some duration using an animation curve. There are variety of different animation curves

we can use to update the value:

• Animated.timing - This animates a value using an easing curve over a specified duration.

E.g. Animated.timing(this.buttonOpacity, { toValue: 1, duration: 500, easing:

Easing.inOut(Easing.ease) }). The Easing API provides most of the common easing

curve functions used for animation. If we don’t provide a value for easing, the default is

Easing.inOut(Easing.ease), which generally looks pretty good.

• Animated.spring - This animates a value using a simulated spring. The tension and friction

of the spring are customizable. E.g. Animated.spring(this.buttonOpacity, { toValue: 1,

friction: 7, tension: 40 }).

• Animated.decay - This animates a value by simulating motion and kinetic energy. The ani-

mated value will decay to 0, using the provided velocity and deceleration. E.g. Animated.decay(this.buttonOpacity,

{ velocity: 1, deceleration: 0.997 }).

Both Animated.timing and Animated.spring accept a delay option which will delay when the

animation begins. We’ll use this delay to stagger the animations.

Calling any of these animation functions returns an object with two methods:

Animation 402

• start(completionCallback?) - We must always start() an animation when we want it to

run (which is often immediately). We can optionally provide a callback that’s invoked when

the animation completes (or is stopped).

• stop() - We can force an animation to stop at any time by calling stop(). If we passed a

completionCallback to the start function, it will be invoked with { finished: false }.

We’ll use Animated.timing for our animations, configuring the animations to last for 500 millisec-

onds. We’ll set the first animation to start 500 milliseconds after the LayoutAnimation finishes, and

we’ll set the second animation to start 1000 milliseconds after.

puzzle/screens/Start.js

async componentDidMount() {

await sleep(500);

await configureTransition(() => {

this.setState({ transitionState: State.WillTransitionIn });

});

Animated.timing(this.toggleOpacity, {

toValue: 1,

duration: 500,

delay: 500,

useNativeDriver: true,

}).start();

Animated.timing(this.buttonOpacity, {

toValue: 1,

duration: 500,

delay: 1000,

useNativeDriver: true,

}).start();

}

Any time we configure an animation, we need to start it with .start(). In this case, we do it right

after calling Animated.timing.

If at any point you run an app and an animation doesn’t work, double check that you’re

calling .start()! It’s very easy to forget.

We’ll come back to useNativeDriver shortly.

Animation 403

Lastly, we need to update the render method to use these animated values. We use animated values

in the style prop of our components, e.g. style={{ opacity: this.buttonOpacity }}, just like we

would if we were using a primitive value directly.

Using an Animated.Value within a component’s style only works if we’re using Animated.View,

Animated.Text, Animated.Image, Animated.ScrollView, or a component created by calling Animated.createAnimatedComponent(Component)

with an existing component. For this reason, we’ll change some of our View components to

Animated.View now that we’re using styles that contain an Animated.Value.

puzzle/screens/Start.js

// ...

render() {

// ...

const toggleStyle = { opacity: this.toggleOpacity };

const buttonStyle = { opacity: this.buttonOpacity };

return (

</View>

{transitionState !== State.Launching && (

<Animated.View style={toggleStyle}>

<Toggle

options={BOARD_SIZES}

value={size}

onChange={onChangeSize}

</Animated.View>

)}

{transitionState !== State.Launching && (

<Animated.View style={buttonStyle}>

</Animated.View>

)}

</View>

);

}

// ...

Animation 404

Try it out

Save Start.js and reload the app. Now the logo should animate, then the toggle buttons, then finally

the start button at the bottom.

The end state should look the same as it did before:

Animated value performance

These animations should probably look pretty smooth on your device. Let’s briefly discuss how they

work under the hood.

Normally when we want to change how a component is rendered, we change either the props or

state of the component, and this triggers a re-render. However, re-rendering 60 times per second

(to achieve 60fps) is often too slow. Thus, animations must happen outside of the lifecycle of our

components.

You might have noticed that we didn’t store our Animated.Value instances in the state of our Start

component. The Animated.Value is a reference to a class instance that wraps a primitive value.

When we change the underlying primitive value with Animated.timing and start, this doesn’t

change the Animated.Value reference. Even if we were to store the animated value in state, the

component wouldn’t re-render when we call Animated.timing or start.

Animation 405

Animated values can be stored in component state, even though they will never trigger a re-

render. By convention, animated values are either stored in state or as instance properties.

Both are common conventions – it’s up to you which you prefer.

When we call Animated.start, the JavaScript animation driver (the thing that actually runs our

animations) uses requestAnimationFrame to update our component’s styles in a way that doesn’t

require a component to re-render (called setNativeProps) every frame.

You may have noticed that we also added useNativeDriver to our animation configurations. This

option is purely for improved animation performance. This option tells React Native to perform

the animation solely on the native thread using the native animation driver, rather than passing

values back and forth between JavaScript and React Native. We can only use useNativeDriver when

animating styles that don’t affect layout, such as opacity, backgroundColor, or transform. We can’t

use useNativeDriver when animating width or top, for example, since these affect layout.

Advanced Animated.Value

Now that we’ve covered the basics of Animated.Value, we can try some more advanced animations.

Let’s replace the placeholder buttons that sit below the logo on the Start screen with some that look

nicer:

Animation 406

So far when we’ve created buttons in this book, we’ve used TouchableOpacity and TouchableHighlight.

The animations for these are pre-defined to change the opacity and background color of the button’s

view. But what if we want a custom button-press animation? In this section, we’ll use the Animated

API to animate the border color, text color, and scale of our own custom Button component.

Updating our buttons

We’re already rendering this Button in our Start screen, and it’s used within the Toggle component,

which we also render from Start. We’ll make the height, color, font size, and border radius of the

button configurable so that we can use the same component in both places.

Open up components/Button.js. Once again, the import statements, propTypes, and styles have

been filled out for you. There’s also a render method that was used to render our placeholder buttons,

but we’ll replace that entirely.

Let’s begin by writing the constructor. Here, we’ll track whether or not the button is currently

pressed within the component’s state. We use the disabled prop to determine if the component is

currently selected (in our toggle component) or not. We’ll also initialize a new Animated.Value to

represent the color and scale of the button, which we’ll store as this.value:

puzzle/components/Button.js

constructor(props) {

super(props);

const { disabled } = props;

this.state = { pressed: false };

this.value = new Animated.Value(getValue(false, disabled));

}

You’ll notice we call a utility function, getValue, to get the initialize the primitive value wrapped

by an Animated.Value. This function has already been written for you at the top of the file:

puzzle/components/Button.js

const getValue = (pressed, disabled) => {

const base = disabled ? 0.5 : 1;

const delta = disabled ? 0.1 : 0.3;

return pressed ? base - delta : base;

};

This utility function returns a different number depending on the state and props of the component.

The exact numbers were chosen arbitrarily to make the animation look nice, but let’s go into a little

more detail about what these will be used for.

Animation 407

When we used Animated.Value for opacity previously (our components below the logo), we used a

value between 0 and 1 to represent the opacity. We were able to use the Animated.Value directly,

since opacity is also a number between 0 and 1. In this case, however, we want to animate a color

value, rather than a number. An Animated.Value always wraps a primitive number value, but we can

use the interpolate method of an animated value to interpolate the number into a different range

(including a range of colors!). For example, we could use 0 to represent black and 1 to represent

white, and interpolate any value between 0 and 1 into a gray color in the range between black and

white.

For this component, we’ll be using a number between 0.1 and 1 to represent the various visual states

of our button. The exact number isn’t important, since we’re mapping it into a different range. In

this case, lower numbers will render our component smaller and dimmer, and higher numbers will

render it bigger and brighter – but we could’ve made it so that higher numbers render the button

smaller.

We want to animate the button whenever the

disabled

prop or the

pressed

state changes. In order

to make the button look and feel good, we’ll animate it to a slightly different size and color for

each combination of disabled and pressed, and we’ll call getValue to get the correct value for each

combination.

The utility function is useful since we don’t need to remember which values to use for each

combination of disabled and pressed. Since getValue is a pure function, we’ll always get

the same result for any given pressed and disabled arguments we pass.

Next, we’ll create a utility method to update this Animated.Value whenever the component’s state

or props change. We want to start a new Animated.timing animation whenever disabled or pressed

change, and we’ll use getValue to determine the desired end value of our Animated.Value for the

animation:

puzzle/components/Button.js

updateValue(nextProps, nextState) {

if (

this.props.disabled !== nextProps.disabled ||

this.state.pressed !== nextState.pressed

) {

Animated.timing(this.value, {

duration: 200,

toValue: getValue(nextState.pressed, nextProps.disabled),

easing: Easing.out(Easing.quad),

}).start();

}

We want to run this animation every time the component will update or receive new props:

Animation 408

puzzle/components/Button.js

componentWillUpdate(nextProps, nextState) {

this.updateValue(nextProps, nextState);

}

componentWillReceiveProps(nextProps, nextState) {

this.updateValue(nextProps, nextState);

}

We’ll also create functions for updating the pressed state, which we’ll soon use in the render method:

puzzle/components/Button.js

handlePressIn = () => {

this.setState({ pressed: true });

};

handlePressOut = () => {

this.setState({ pressed: false });

};

Rendering with Animated.Value

Now we’re ready to render our button.

We’ll start by rendering a TouchableWithoutFeedback to handle touches. This is similar to TouchableOpacity

and TouchableHighlight, except that there’s no visual feedback for the user when tapped. That’s

what we want in this case, since we’re defining our own visual feedback with the Animated API.

In addition to the onPress prop that we’ve used with TouchableOpacity, the TouchableWithoutFeedback

components allows us to pass props that handle the pressing down and releasing of the button

separately. The onPressIn prop fires when we touch the button, and the onPressOut prop fires

when we release the button. We can use these to update the visual/animation state of the button

by updating pressed in state. It’s best to use onPressIn and onPressOut for visual feedback, and to

continue using onPress for the behavior of a button (calling the onPress function prop passed into

Button).

We want to render a TouchableWithoutFeedback, passing it an onPress, onPressIn, and onPressOut

prop. Go ahead and delete the existing render method (we don’t need it anymore), and replace it

with the following one:

Animation 409

puzzle/screens/Start.js

// ...

render() {

const {

props: { title, onPress, color, height, borderRadius, fontSize },

} = this;

// ...

return (

<TouchableWithoutFeedback

onPress={onPress}

onPressIn={this.handlePressIn}

onPressOut={this.handlePressOut}

{/* ... */}

</TouchableWithoutFeedback>

);

}

// ...

Note that we propagate the onPress prop from our Button component into the TouchableWithoutFeedback.

Within the TouchableWithoutFeedback, we’ll render an Animated.View and an Animated.Text

component:

puzzle/screens/Start.js

// ...

render() {

// ...

return (

<TouchableWithoutFeedback

onPress={onPress}

onPressIn={this.handlePressIn}

onPressOut={this.handlePressOut}

<Animated.View style={/* ... */}>

<Animated.Text style={/* ... */}>

{title}

Animation 410

</Animated.Text>

</Animated.View>

</TouchableWithoutFeedback>

);

}

// ...

For both the Animated.View and Animated.Text, we want to use this.value to modify their color

– we can do this with this.value.interpolate. As mentioned previously, the interpolate method

lets us interpolate an Animated.Value into a different range. In this case, we’ll interpolate a number

between 0 and 1 into a color between black and the color prop, where 0 represents black and 1

represents fully colored. In render(), let’s make a new variable above the return statement to hold

this color:

puzzle/components/Button.js

const animatedColor = this.value.interpolate({

inputRange: [0, 1],

outputRange: ['black', color],

});

We can use the output of this.value.interpolate as a style attribute for any attribute that accepts a

color. Similarly, we’ll interpolate this.value into a suitable range to update the scale of the button:

puzzle/components/Button.js

const animatedScale = this.value.interpolate({

inputRange: [0, 1],

outputRange: [0.8, 1],

});

In this case, 0 will map to 0.8 and 1 will map to 1. These numbers were chosen by testing the

animation and trying a few different values to see what looked best.

Next we’ll create a style object for our Animated.View:

Animation 411

puzzle/components/Button.js

const containerStyle = {

borderColor: animatedColor,

borderRadius,

height,

transform: [{ scale: animatedScale }],

};

Transform

By using the transform attribute of a component’s style, we can apply a transformation matrix to

the component before rendering it. This allows us to scale, rotate, translate, or skew the component.

Transformations applied this way don’t affect the layout of other components. Even if we make a

component bigger by setting a { scale: 2 } transformation, the sibling and parent components

will remain in the exact same position. If components overlap after a transformation is applied, the

component rendered last will render on top (unless zIndex is used for re-ordering).

Because transform doesn’t affect the layout of our components, we’re able to animate it with

useNativeDriver for improved performance. For this reason, we generally use transform whenever

we can for animations. Rather than animating top or left, we can translate. Rather than animating

width and height, consider a scale transformation. Some of the time we’ll still need to animate

layout properties, but it’s good to consider whether a transform can accomplish the same thing.

If you’re coming from CSS, you might be familiar with the transform attribute already. The idea

is the same, although the API is fairly different. The API in React Native might seem unusual: the

transform attribute takes an array of objects, each with a single key. This object-based description

of transformations, e.g. { scale: 2 }, lets React Native expose a type-safe API when used with

the Flow language. By contrast, the API in CSS is string-based, e.g. scale(2), which can’t be fully

type-checked in Flow. We can’t simplify the API by combining the array of objects into a single

object, since it’s possible to apply several of the same kind of transformation, e.g. [{ scale: 2 },

{ rotateX: '45deg' }, { scale: 1.2 }].

To see all the transformations you can apply to components, check out the React Native documen-

tation

https://facebook.github.io/react-native/docs/transforms.html

And a style object for our Animated.Text:

Animation 412

puzzle/components/Button.js

const titleStyle = {

color: animatedColor,

fontSize,

};

Update the return value as follows:

puzzle/components/Button.js

return (

<TouchableWithoutFeedback

onPress={onPress}

onPressIn={this.handlePressIn}

onPressOut={this.handlePressOut}

<Animated.View style={[styles.container, containerStyle]}>

<Animated.Text style={[styles.title, titleStyle]}>

{title}

</Animated.Text>

</Animated.View>

</TouchableWithoutFeedback>

);

Remember to use wrapped components like Animated.View when working with animations!

It’s very easy to forget. If you pass an Animated.Value in the style of a normal View, you’ll

get seemingly-unrelated error messages that are hard to decipher.

Try it out

Save Button.js. Once the app reloads, you should see the button component we just wrote on the

Start screen. We use it both for choosing the size of the puzzle and for the “Start Game” button:

Animation 413

If you try tapping the button, you should see the border color, text color, and scale animate when

you press and when you release your finger.

Starting the game

The last thing we need to do on the start screen is enable the transition to the game screen. We’ll

transition when the user taps the “Start Game” button at the bottom of the screen.

Open up Start.js again. We’re going to add a function handlePressStart that sets the transitionState

to WillTransitionOut, and configures a LayoutAnimation using the configureTransition utility

function we wrote earlier. After the animation completes we’ll call the onStartGame prop, which

tells the App to unmount this screen and render the game screen instead. Add the following function

to the Start component:

Animation 414

puzzle/screens/Start.js

handlePressStart = async () => {

const { onStartGame } = this.props;

await configureTransition(() => {

this.setState({ transitionState: State.WillTransitionOut });

});

onStartGame();

};

Then we’ll update the render method with two new things.

• We’ll pass the handlePressStart function to our Button component’s onPress prop.

• If we’re in the WillTransitionOut state, we don’t want to render anything. This will cause

the components on the screen to fade out, due to the LayoutAnimation we configured.

In order to achieve this, we’ll only render our components when transitionState !==

State.WillTransitionOut.

Update the render method to the following:

puzzle/screens/Start.js

render() {

const { size, onChangeSize } = this.props;

const { transitionState } = this.state;

const toggleStyle = { opacity: this.toggleOpacity };

const buttonStyle = { opacity: this.buttonOpacity };

return (

transitionState !== State.WillTransitionOut && (

</View>

{transitionState !== State.Launching && (

<Animated.View style={toggleStyle}>

<Toggle

options={BOARD_SIZES}

value={size}

onChange={onChangeSize}

Animation 415

</Animated.View>

)}

{transitionState !== State.Launching && (

<Animated.View style={buttonStyle}>

</Animated.View>

)}

</View>

)

);

}

Try it out

Save Start.js. Once the app reloads, tap the “Start Game” button. You should see the components

on this screen fade out.

Wrapping up the Start screen

We’ve successfully created the start screen for our puzzle game. To do this we used two types of

animations:

• LayoutAnimation - These animations automatically transition our UI between two states. This

is especially useful when animating many components and style attributes at once, or when

we don’t know the exact pixel values to use in the animation (e.g. with flex-based layouts). We

can only animate layout attributes, such as width or flex. If we want to animate non-layout

attributes like color, we’ll need to use Animated.

• Animated - These animations offer us more control than LayoutAnimation, but also have a

more complex API. We have to specify the exact starting and ending value for each animation,

and start each animation at the appropriate time. We use these animations when animating

multiple components independently, or when animating non-layout attributes like color. We

can use useNativeDriver to improve the performance of our non-layout animations.

Next, we’ll use these same animation techniques to make the Game screen.

Building the Game screen

The Game screen shows the current puzzle, along with a preview of the completed puzzle in the top

left and a timer and moves counter in the top right:

Animation 416

Game lifecycle

We’re going to combine a lot of different animations to show and hide the game screen. Since these

are all asynchronous and will span multiple components, the code can easily get complicated without

careful planning. It’s important that we have a clear understanding of how the animations should

work before we attempt to code them.

Let’s think about the “lifecycle” of the Game screen. There are two phases of the lifecycle: the

“transition in” phase where the screen transitions into view, and the “transition out” phase where it

transitions out of view.

The Game screen renders the Board component as a child. The Board handles a lot of the animations.

In the “transition in” phase, we must first display the Game before we display the Board. In the

“transition out” phase, we must hide the Board before we hide the Game – we do this to give the

board time to animate before unmounting it.

The “transition in” phase

Let’s consider the following diagram of the “transition in” phase:

Animation 417

Here’s what needs to happen at each step:

1. After the user presses the start button on the Start screen, the Start screen fades out, and the App

renders the Game screen. The App passes an image and a puzzle state as props to the Game.

The possible states of the Game screen are:

puzzle/screens/Game.js

const State = {

LoadingImage: 'LoadingImage',

WillTransitionIn: 'WillTransitionIn',

RequestTransitionOut: 'RequestTransitionOut',

WillTransitionOut: 'WillTransitionOut',

};

The Game begins in either the LoadingImage or WillTransitionIn state.

2. Since the image may not have fully downloaded yet, it may be null to begin with. If that’s the

case, the Game begins in the LoadingImage state and renders an ActivityIndicator until the image

loads:

Animation 418

3. Otherwise, the

Game

begins in the

WillTransitionIn

state. In this state, the

Game

will render the

top and bottom of the screen, along with an empty game board in the middle:

Animation 419

4. After the

WillTransitionIn

state, it’s the

Board

component’s turn to animate things. The

Board

component handles animating the puzzle pieces into view:

Animation 420

The possible states of the

Board

component are:

puzzle/components/Board.js

const State = {

WillTransitionIn: 'WillTransitionIn',

DidTransitionIn: 'DidTransitionIn',

DidTransitionOut: 'DidTransitionOut',

};

The Board begins in the WillTransitionIn state, and starts animating each puzzle piece from

beneath the screen into the center of the screen. Once these animations finish, it will enter the

DidTransitionIn state and call its onTransitionIn prop (passed in from Game).

5. The Game listens the Board to call onTransitionIn. Once it’s called, the Game will start the timer

in the top right. Now the game has begun!

The “transition out” phase

Now let’s consider how the Game and Board transition out of view:

Animation 421

1. Once the user completes the puzzle or presses the quit button, the game enters the RequestTransitionOut

state. In this state, the Game tells the Board to do any cleanup it needs by passing the prop

teardown={true}.

2. Upon receiving the teardown prop, the Board transitions animates each puzzle piece out of

view. Once this cleanup is done, the Board will transition to the DidTransitionOut state and

call onTransitionOut. We do this to give the Board a chance to animate before we unmount the

component:

3. When the onTransitionOut prop is called, the Game transitions to its final state,WillTransitionOut.

Animation 422

In the WillTransitionOut state, the Game will fade out the entire UI in preparation for displaying

the Start screen again.

Transitioning in and out

Now that we have a plan, we can start our implementation! Open screens/Game.js. Once again,

some of the skeleton of the screen has already been written for you.

You can ignore the handlePressSquare function for now; we’ll come back to that later.

Let’s begin by writing the constructor for the Game screen. Here we’ll use the existence of the image

prop to determine whether we should begin in the LoadingImage or WillTransitionIn state. We’ll

run our configureTransition utility to enqueue a LayoutAnimation on initial render. This covers

step 1 of the “transition in” phase described previously.

Open up Game.js and add the following constructor:

puzzle/screens/Game.js

constructor(props) {

super(props);

const { image } = props;

this.state = {

transitionState: image ? State.WillTransitionIn : State.LoadingImage,

moves: 0,

elapsed: 0,

previousMove: null,

image: null,

};

configureTransition();

}

If the game begins in the LoadingImage state, then we want to watch for when the image prop changes

so we know when to transition to the WillTransitionIn state. This is step 2 of the “transition in”.

Add the following componentWillReceiveProps to do this:

Animation 423

puzzle/screens/Game.js

componentWillReceiveProps(nextProps) {

const { image } = nextProps;

const { transitionState } = this.state;

if (image && transitionState === State.LoadingImage) {

configureTransition(() => {

this.setState({ transitionState: State.WillTransitionIn });

});

}

We can update the render method to handle these first few states. If we’re still loading an image (Game

is in the LoadingImage state) we want to show an ActivityIndicator. If we’ve finished loading the

image, we want to show the stats and image preview at the top of the screen. Let’s update our render

method to the following:

puzzle/screens/Game.js

// ...

render() {

const { puzzle, puzzle: { size }, image } = this.props;

const { transitionState, moves, elapsed, previousMove } = this.state;

return (

{transitionState === State.LoadingImage && (

)}

{transitionState !== State.LoadingImage && (

</View>

{/* ... */}

</View>

)}

</View>

);

}

Animation 424

// ...

Try it out

Save Game.js and reload the app. If you tap the “Start Game” button, you should see the start screen

fade out and the first few components of the game screen fade in.

Transitioning in and out, continued

Now let’s add the game board. The Board component will handle its own transitions once rendered

from Game. Rendering the Board completes step 3 of the “transition in” phase, and step 4 will

be completed within the Board component. Let’s create the methods we need to pass as props

to the Board component. We’ll start by creating a method for handling when the Board finishes

transitioning.

The timer in the top right of the screen counts how much time has elapsed since the game started.

Once the board fully transitions in, it will call its onTransitionIn prop. That’s when we want

to start the timer. We’ll use setInterval to increment state.elapsed every second. Let’s add a

handleBoardTransitionIn method that we can pass to onTransitionIn to do this:

Animation 425

puzzle/screens/Game.js

handleBoardTransitionIn = () => {

this.intervalId = setInterval(() => {

const { elapsed } = this.state;

this.setState({ elapsed: elapsed + 1 });

}, 1000);

};

This completes the last step of the transition in phase. However, we’ll want to add a few more things

before we update the render method. Let’s consider how the Game screen should transition out.

The game ends either when the puzzle is finished or when the user presses the “Quit” button. In

both of these scenarios, we want to enter the transitionState called RequestTransitionOut. In

this state, we give the Board time to run its transition animation.

Let’s add a requestTransitionOut function to Game that handles updating transitionState. We also

want to stop the timer in the top right once we call this:

puzzle/screens/Game.js

requestTransitionOut = () => {

clearInterval(this.intervalId);

this.setState({ transitionState: State.RequestTransitionOut });

};

Pressing the quit button should also set the transitionState to RequestTransitionOut. We can call

the same requestTransitionOut function for this. When the quit button is pressed, we’ll handle

it with a new function handlePressQuit. We’ll use the Alert API, which we covered in the “Core

APIs” chapter, to ask the user if they are sure they want to quit. Add the following function to Game:

puzzle/screens/Game.js

handlePressQuit = () => {

Alert.alert(

'Quit',

'Do you want to quit and lose progress on this puzzle?',

[

{ text: 'Cancel', style: 'cancel' },

{

text: 'Quit',

style: 'destructive',

onPress: this.requestTransitionOut,

Animation 426

);

};

That handles step 1 of the “transition out” phase. The Board component will handle step 2 internally.

Once the board transitions out (remember, we had to give it time to transition before unmounting),

it will call its onTransitionOut prop. When this happens, we want to fade out the components

on this game screen. We can do that using our configureTransition utility function. After

that we want to call the onQuit() prop, which will return us to the start screen. Let’s write a

handleBoardTransitionOut function which we’ll pass to onTransitionOut:

puzzle/screens/Game.js

handleBoardTransitionOut = async () => {

const { onQuit } = this.props;

await configureTransition(() => {

this.setState({ transitionState: State.WillTransitionOut });

});

onQuit();

};

That completes the “transition out” phase! Now we can update the render method to render both

the Board and the “Quit” Button.

Before returning anything, we’ll first check if we’re in the WillTransitionOut state – if we are,

we don’t want to render anything, since we want our LayoutAnimation to fade the entire screen

out. We also need to remember to pass the teardown prop to the Board when Game is in the

RequestTransitionOut state.

Let’s update the render method to the following:

puzzle/screens/Game.js

render() {

const { puzzle, puzzle: { size }, image } = this.props;

const { transitionState, moves, elapsed, previousMove } = this.state;

return (

transitionState !== State.WillTransitionOut && (

{transitionState === State.LoadingImage && (

)}

Animation 427

{transitionState !== State.LoadingImage && (

</View>

<Board

puzzle={puzzle}

image={image}

previousMove={previousMove}

teardown={transitionState === State.RequestTransitionOut}

onMoveSquare={this.handlePressSquare}

onTransitionOut={this.handleBoardTransitionOut}

onTransitionIn={this.handleBoardTransitionIn}

</View>

)}

</View>

)

);

}

Try it out

Save Game.js and reload the app. If you tap the “Start Game” button, you should see the start screen

fade out and the game screen fade in:

Animation 428

The board won’t render yet, but we’ll add that in the next chapter. If you tap the “Quit” button, you

should see a dialog with options to “Cancel” or “Quit”:

Animation 429

These won’t do anything yet though, since the

Board

will never call its

onTransitionOut

prop.

That completes the Game screen, for now. In the next chapter, we’ll build the Board.

Summary

In this chapter, we used the two main animation APIs included in React Native: Animated and

LayoutAnimation. We used LayoutAnimation to move components around the screen, and to fade

components into and out of view. We used Animated to animate non-layout styles like colors and

when we wanted more control over individual animations.

In the next chapter (“Gestures”), we’ll finish our puzzle game. We’ll build the Board component

and allow the user to rearrange puzzle pieces by dragging. Gestures rely heavily on Animated, so

we’ll see a few more ways we can use the Animated API, while ensuring smooth, high-performance

animations.

Gestures

Gestures are fundamental to building mobile apps. Well-designed gestures can make mobile apps

feel intuitive and easy to use. Just like with animations, we often use gestures to imitate movement

in the real world or to build interactivity similar to familiar physical objects. For this reason, most

gestures are accompanied by animations – physical objects rarely teleport from one place to another,

so neither should components in our UI. We can leverage the Animated API from the previous chapter

to build gestures that feel natural.

Simple gestures are supported out-of-the-box by React Native components. When we want to add

a tap gesture, we can use TouchableOpacity or TouchableHighlight. For more advanced gestures,

however, we’ll need to create our own components using lower-level APIs. In this chapter we’ll

explore gestures by adding an interactive game board to the puzzle app we started in the previous

chapter.

Picking up where we left off

We successfully built the start screen and we started the game screen for our puzzle app. Next, we’re

going to add the interactive board.

This is a code checkpoint. If you haven’t been coding along with us but would like to start

now, we’ve included a snapshot of our current progress in the sample code for this book.

First, copy the puzzle/2 directory to somewhere else on your computer. Then in the terminal,

you can navigate to the directory you copied and install the dependencies by running yarn.

For example, if you unzipped the book’s sample code to

∼

/Downloads/fsrn/

, on a macOS

or Linux computer you would run:

$ cp -r ~/Downloads/fsrn/puzzle/2 ~/Downloads/puzzle

$ cd ~/Downloads/puzzle

$ yarn

Building the board

Our Board component is responsible for animating the puzzle pieces into view when the components

mount, handling the drag gesture as the user moves the pieces, and animating the pieces out of view

when the game ends.

Gestures 431

The Board component has already been started for you. Let’s take a look at what’s already there.

Open Board.js.

Transition states

Just like with the Start and Game screens we built in the previous chapter, we’ll use a transitionState

prop to control the various transitions in the Board. These are the states of the board:

puzzle/components/Board.js

const State = {

WillTransitionIn: 'WillTransitionIn',

DidTransitionIn: 'DidTransitionIn',

DidTransitionOut: 'DidTransitionOut',

};

The Board begins in the WillTransitionIn state, and starts animating each puzzle piece from

beneath the screen into the center of the screen:

Once these animations finish, it will enter the DidTransitionIn state and call its onTransitionIn

prop (passed in from Game). At this point, the puzzle pieces become interactive:

Gestures 432

The

Game

will tell the board when the game is finished and it’s time to cleanup by passing the

teardown prop. Upon receiving the teardown prop, the Board transition animates each puzzle piece

out of view:

Gestures 433

Once this cleanup is done, the

Board

will transition to the

DidTransitionOut

state and call

onTransitionOut.

Board props

Next lets look at the propTypes for this component:

puzzle/2/1/components/Board.js

static propTypes = {

puzzle: PuzzlePropType.isRequired,

teardown: PropTypes.bool.isRequired,

image: Image.propTypes.source,

previousMove: PropTypes.number,

onMoveSquare: PropTypes.func.isRequired,

onTransitionIn: PropTypes.func.isRequired,

onTransitionOut: PropTypes.func.isRequired,

};

The board is passed the current state of the puzzle, the image, and the previousMove. From these,

the board can determine how to render the puzzle. The board will never modify the state of the

Gestures 434

puzzle – instead, the board will call onMoveSquare to inform the Game component that a piece has

been moved.

We use a PropTypes.shape to validate the fields within the puzzle object:

puzzle/validators/PuzzlePropType.js

import PropTypes from 'prop-types';

export default PropTypes.shape({

size: PropTypes.number.isRequired,

empty: PropTypes.number.isRequired,

board: PropTypes.arrayOf(PropTypes.number.isRequired).isRequired,

});

The puzzle object contains the size of the board, the arrangement of pieces on the board, and an

indicator recording which piece is the empty piece. Each piece is represented by a number. In its

finished state, the piece numbers will be properly sorted from small to large within board. In other

words, the number represents the “correct” or “final” position of the piece in the completed puzzle.

The empty value refers to the number of a piece (not to an index in the board array).

If we overlay these numbers on top of the puzzle board, we can see how each number corresponds

with a piece:

Gestures 435

For this example, the initial state of a puzzle object is:

{

size: 3,

empty: 8,

board: [7, 3, 1, 6, 8, 2, 0, 4, 5]

}

When completed, the puzzle object will be:

{

size: 3,

empty: 8,

board: [0, 1, 2, 3, 4, 5, 6, 7, 8]

}

We’ll use the piece numbers directly when rendering the board. We’ll use utility functions for

determining anything else from the puzzle object.

Gestures 436

If you recall from the previous chapter, each puzzle uses a random image fetched from a remote API.

When we render the puzzle, we’ll need to “split up” the image into a grid of puzzle pieces.

We won’t actually modify the raw image data – instead we’ll render the same image multiple times,

once for each piece, and offset the image’s position. We can use a style with overflow: hidden to

hide the excess parts of the image we don’t want to show. We’ll use the piece’s number to calculate

the position of the image for that piece.

You’ll notice at the top of the file we import two utility functions:

import { availableMove, getIndex } from '../utils/puzzle';

We’ll use availableMove to determine which directions the user may drag any given piece. And

we’ll use getIndex to determine the current position of any given piece.

The other props, teardown, onTransitionIn, and onTransitionOut, are all used to communicate state

changes between the Game and Board components.

Initializing the board

We’ll start by writing a simplified version of the game board where the pieces don’t move. After we

have the pieces showing up in the correct positions, we’ll add the animation and gestures.

Each piece on the board will use an Animated.Value to represent its top, left, and scale. This

gives us fine-grained control over the animations of each piece. We can use a helper function,

calculateItemPosition, already imported at the top of the file to determine the correct starting

top and left position of each piece.

Our constructor needs to do 2 things:

• Initialize the transitionState to WillTransitionIn

• Create an Animated.Value for the top, left, and scale of each piece

Add the following constructor to components/Board.js:

Gestures 437

puzzle/2/1/components/Board.js

constructor(props) {

super(props);

const { puzzle: { size, board } } = props;

this.state = { transitionState: State.WillTransitionIn };

this.animatedValues = [];

board.forEach((square, index) => {

const { top, left } = calculateItemPosition(size, index);

this.animatedValues[square] = {

scale: new Animated.Value(1),

top: new Animated.Value(top),

left: new Animated.Value(left),

};

});

}

Recall from the previous chapter that an Animated.Value wraps a number. We need to

instantiate a separate Animated.Value for the top, left, and scale of each puzzle piece

on the board, since we want to animate all of these values independently.

We’ll render each puzzle piece with an absolute position, so that it renders at the top-left of the

board. Then we’ll use the top and left animated values to position the piece relative to the top-left

of the board.

Now that we have our constructor, we’ll also need a componentDidMount method where we:

• Start the initial animation (where the puzzle pieces fly onto the board)

• Set transitionState to DidTransitionIn once the animation completes

• Call onTransitionIn to inform the Game that the transition animation has completed and the

game has begun

We’ll handle starting the transition animation later in the chapter, so for now, let’s add a

componentDidMount method that sets the transitionState and calls onTransitionIn:

Gestures 438

puzzle/2/1/components/Board.js

async componentDidMount() {

const { onTransitionIn } = this.props;

this.setState({ transitionState: State.DidTransitionIn });

onTransitionIn();

}

Rendering the board

Next, let’s render each piece on the game board. In order to determine the proper size of the board

and each piece, we’ll use two utility functions that have already been imported at the top of the file:

• calculateContainerSize() - This function returns the size to render the board, in pixels. Since

the board is a square, we’ll use this size for both the width and height.

• calculateItemSize(size) - This function uses puzzle.size to divide the board into an even

number of rows and columns. We’ll use the returned pixel size for the width and height of

each piece.

We’ll represent the board with a View. We’ll map each puzzle piece in puzzle.board into an

Animated.View that contains an Image.

Add the following render method to components/Board.js:

puzzle/2/1/components/Board.js

render() {

const { puzzle: { board } } = this.props;

const { transitionState } = this.state;

const containerSize = calculateContainerSize();

const containerStyle = { width: containerSize, height: containerSize };

return (

{transitionState !== State.DidTransitionOut &&

board.map(this.renderSquare)}

</View>

);

}

Gestures 439

Notice that we map each piece in the board through this.renderSquare. Let’s write the renderSquare

method now. This method is called with two arguments:

• square - The numeric value of the piece in puzzle.board

• index - The index of the square within the puzzle.board array

In other words, the square represents the “correct” position of the puzzle piece (within the original

image), while the index represents the current position of the puzzle piece (within the rearranged

image).

When the board is in the DidTransitionOut state, we don’t render any pieces. This shouldn’t

be necessary, since the pieces should have already animated off-screen. However, there’s a

bug that occurs when combining Animated and useNativeDriver that causes the pieces to

render without their transform styles.

Let’s write renderSquare now. We’ll start by declaring the method and destructuring the props and

state we’ll need:

puzzle/2/1/components/Board.js

renderSquare = (square, index) => {

const { puzzle: { size, empty }, image } = this.props;

const { transitionState } = this.state;

If the square is the empty square of the puzzle (puzzle.empty), then we shouldn’t render it:

puzzle/2/1/components/Board.js

renderSquare = (square, index) => {

const { puzzle: { size, empty }, image } = this.props;

const { transitionState } = this.state;

if (square === empty) return null;

Next, we’ll call calculateItemSize to get the pixel size of the puzzle piece. This value will be the

same for every piece:

Gestures 440

puzzle/2/1/components/Board.js

if (square === empty) return null;

const itemSize = calculateItemSize(size);

We can use the itemSize to create a style, itemStyle, for the Animated.View that we’ll render. The

view should have a width and height equal to itemSize, and use a transform to correctly position

it on the board. This is where the Animated.Value array we set up earlier comes in:

puzzle/2/1/components/Board.js

const itemSize = calculateItemSize(size);

const itemStyle = {

position: 'absolute',

width: itemSize,

height: itemSize,

overflow: 'hidden',

transform: [

{ translateX: this.animatedValues[square].left },

{ translateY: this.animatedValues[square].top },

{ scale: this.animatedValues[square].scale },

};

Note that we use a transform with translateX and translateY, instead of left and

top. If you recall from the previous chapter, this allows us to animate these values with

useNativeDriver for improved performance. In this chapter, we use the names top and left

to refer to a piece’s position for simplicity, even though we’re actually setting the translateX

and translateY.

Within the Animated.View that uses this style, we’ll render the image of the puzzle piece. With some

clever math, we can offset the image to display the correct portion for each piece:

Gestures 441

puzzle/2/1/components/Board.js

const imageStyle = {

position: 'absolute',

width: itemSize * size + (itemMargin * size - 1),

height: itemSize * size + (itemMargin * size - 1),

transform: [

{

translateX: -Math.floor(square % size) * (itemSize + itemMargin),

{

translateY: -Math.floor(square / size) * (itemSize + itemMargin),

};

The exact calculations here and elsewhere in the chapter are specific to this game, so we

won’t cover them in much detail. However, animation and gesture code in general tends to

rely on manual calculations, so you may find it useful to try to understand the calculations

and utility functions in this chapter.

Lastly, we can put everything together by rendering an Animated.View and an Image:

puzzle/2/1/components/Board.js

return (

<Animated.View key={square} style={itemStyle}>

</Animated.View>

);

};

Try it out!

Save Board.js. After the app reloads, press the start button, and you should see the board fade in!

Gestures 442

Making pieces draggable

Now that we’re rendering our puzzle pieces, we can focus on making them draggable. In order to

do this, we’ll need to learn how to use the Gesture Responder System.

Gesture Responder System

React Native provides the Gesture Responder System for building complex interactions like drag-

ging. The Gesture Responder System gives us fine-grained control over which components should

receive and respond to touch events.

Each time the user touches the screen, moves their finger, or lifts their finger, the operating system

records an independent event called a “touch event.” Interpreting one or more of these independent

touch events results in a gesture. A tap gesture may consist of the user touching the screen and

lifting their finger immediately. A drag gesture may consist of a user touching the screen, moving

their finger around the screen, and then lifting their finger.

Touch events can interact in complex ways in mobile apps. Imagine a horizontally draggable slider

within a vertical scrollview – how do we determine which finger movements should affect the slider

and which should affect the scrollview? The Gesture Response System gives us a set of callbacks

which help us handle the right touch events from the right component.

Gestures 443

Responder lifecycle

Let’s look at how touch events flow between components in the responder system.

At its core, the responder system determines which view owns the global “interaction lock” at any

given time. When granted the interaction lock, a view is known as the “responder”, since it responds

to touch events. Generally the responder view should show visual feedback, such as highlighting or

moving. While a touch gesture is occuring, the interaction lock may be transferred to an ancestor

view of the responder.

There are function props a view can implement to request the interaction lock:

• View.props.onStartShouldSetResponder: (e) => true - If a touch gesture begins on this

view, should this view become the responder?

• View.props.onMoveShouldSetResponder: (e) => true - If the user moves their finger over

this view during a touch gesture, should this view become the responder?

If one of these functions returns true, then the view has requested the interaction lock. If no other

view currently owns the interaction lock, then the requesting view automatically becomes the re-

sponder. If a view does own the interaction lock, then the owner’s onResponderTerminationRequest

function prop will be called – the owner can decide whether to keep or hand off the interaction lock.

One of the following props will be called for the view requesting the interaction lock:

• View.props.onResponderGrant: (e) => {} - The request for the interaction lock was granted!

The requesting view is now the responder. The view may want to respond to receiving the

interaction lock in some way, e.g. by setting some state that renders a highlight.

• View.props.onResponderReject: (e) => {} - The request for the interaction lock was rejected.

The view that owned the interaction lock refused to give it up.

The responder view’s props will be called as the touch gesture continues:

• View.props.onResponderMove: (e) => {} - This is called whenever the user moves their finger.

• View.props.onResponderRelease: (e) => {} - This is called when the user lifts their finger.

• View.props.onResponderTerminationRequest: (e) => true - Another view has requested the

interaction lock. Return false to retain the lock, or true to hand it off.

• View.props.onResponderTerminate: (e) => {} - The interaction lock has been taken away.

This is generally due to returning true from onResponderTerminationRequest, but there are

cases where the operating system will take the interaction lock without asking (without

onResponderTerminationRequest being called first). For example, onResponderTerminate will

be called upon receiving a phone call – onResponderTerminationRequest will not be called,

since returning false would not prevent the operating system from taking over control of the

screen.

When a touch starts, the onStartShouldSetResponder prop will be called for the inner-most view

containing the touch. If the view returns false (or doesn’t implement the prop), then the parent’s

onStartShouldSetResponder prop will be called. This process continues up the view hierarchy.

Gestures 444

Capture phase

Sometimes a parent view will need to request the interaction lock before any of its children have

a change. In this case, we don’t want the bottom-up calling order of onStartShouldSetResponder.

There are two other props we can use that are called top-down, i.e. first the parent has a chance to

become responder, then the child.

• View.props.onStartShouldSetResponderCapture: (e) => true

• View.props.onMoveShouldSetResponderCapture: (e) => true

These props are analogous to onStartShouldSetResponder and onMoveShouldSetResponder. These

“capture” props are called first starting from the parent, and then down the view hierarchy. If no

view returns true, then onStartShouldSetResponder and onMoveShouldSetResponder will be called

starting from the inner-most view.

The portion of the responder lifecycle where touches are “captured” from top to bottom is called the

capture phase. The portion of the lifecycle where touches are capture from bottom to top is called

the bubble phase.

If you’re coming from web development, you’ll probably be familiar with these terms – this

part of the responder system is modeled after DOM events!

Diagram

The following diagram illustrates the responder lifecycle for new touches:

Gestures 445

The same flow happens every time a touch moves, except that onMoveShouldSetResponder and

onMoveShouldSetResponderCapture are called instead of onStartShouldSetResponder and onStartShouldSetResponderCapture.

Gestures 446

Touch event object

All of the responder function props are called with an event object, often abbreviated as evt or e, e.g.

the argument in onStartShouldSetResponder = (e) => {}. The event object contains the property

nativeEvent which is an object containing:

• locationX - The X position of the touch, relative to the responder view

• locationY - The Y position of the touch, relative to the responder view

• pageX - The X position of the touch, relative to the root view

• pageY - The Y position of the touch, relative to the root view

• timestamp - The time when the touch occurred

• identifier - The id of the touch

• target - The id of the view receiving the touch event

• touches - Array of all current touches on the screen

• changedTouches - Array of all touch events that have changed since the last event

You may need to access these properties to do touch-related calculations. However, React Native

provides a higher-level convenience wrapper on top of the responder system that you’ll likely use

instead.

PanResponder

The touch event object contains raw values, such as the time and location of touches. Many

interactions, however, will require the distance and velocity of the touches over time. React Native

provides a higher-level API called PanResponder.

The PanResponder intercepts each responder function so that it can maintain a gestureState

object containing the distance, velocity, and a few other computed properties. We can create a

PanResponder with PanResponder.create(config), where the config object contains any of the

following functions:

• onStartShouldSetPanResponder: (e, gestureState) => {}

• onStartShouldSetPanResponderCapture: (e, gestureState) => {}

• onMoveShouldSetPanResponder: (e, gestureState) => {}

• onMoveShouldSetPanResponderCapture: (e, gestureState) => {}

• onPanResponderReject: (e, gestureState) => {}

• onPanResponderGrant: (e, gestureState) => {}

• onPanResponderStart: (e, gestureState) => {}

• onPanResponderEnd: (e, gestureState) => {}

• onPanResponderRelease: (e, gestureState) => {}

• onPanResponderMove: (e, gestureState) => {}

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• onPanResponderTerminate: (e, gestureState) => {}

• onPanResponderTerminationRequest: (e, gestureState) => {}

• onShouldBlockNativeResponder: (e, gestureState) => {}

Each of these wraps a responder function (with roughly the same name) and then calls it with

the gestureState object in addition to the original event object. For example, onPanResponderMove

wraps onResponderMove.

The only exception is onShouldBlockNativeResponder, which is a totally different function

from the underlying responder functions. This function is for blocking native components

from becoming the responder, and only works on Android. We won’t be using it.

The gestureState object contains the following properties:

• stateID - The id of the gestureState – persisted as long as there’s at least one touch on screen

• moveX - The latest screen coordinates of the most recently moved touch

• moveY - The latest screen coordinates of the most recently moved touch

• x0 - The screen coordinates at the time the responder was granted

• y0 - The screen coordinates at the time the responder was granted

• dx - Accumulated distance of the gesture since the touch started

• dy - Accumulated distance of the gesture since the touch started

• vx - Current velocity of the gesture

• vy - Current velocity of the gesture

• numberActiveTouches - Number of touches currently on screen

Now that we’ve covered the fundamentals, let’s put them to use!

Draggable component

For our puzzle game, we want to build a dragging gesture. In order to do this, we’ll need to:

• Handle when the user touches a specific puzzle piece component

• Continously monitor the x, y offset as they move their finger

• Record the final x, y offset when they lift their finger

As the user moves their finger, we can monitor the x, y offset to update the puzzle piece component’s

transform. Thus the puzzle piece will follow the user’s finger as it moves across the screen – this is

how we simulate “dragging” in mobile apps. When the user lifts their finger, we will use the final

x, y offset to determine how to update the puzzle object accordingly.

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Creating a PanResponder

Let’s use a PanResponder to make a Draggable component. We’ll use our Draggable component to

handle touches and to monitor the x, y offset of the drag.

We’ll make this component fairly generic, both to better separate the dragging code from the

rendering code, and to make the component easy to reuse. The component’s sole purpose will be

to handle the logic around touch events. It won’t render anything to the UI itself. The Draggable

component will pass a PanResponder and x, y offset to its children – its children will be responsible

for using this information to render the UI. The Draggable component will expose callback props

that notify the parent as touch events occur. We’ll use these events to update the puzzle object.

Open up components/Draggable.js. The skeleton of the component has already been written for

you. Take a look at the propTypes:

puzzle/2/components/Draggable.js

static propTypes = {

children: PropTypes.func.isRequired,

onTouchStart: PropTypes.func,

onTouchMove: PropTypes.func,

onTouchEnd: PropTypes.func,

enabled: PropTypes.bool,

};

This component will render arbitrary children by calling the children function prop. When the user

interacts with the component, it will call the onTouchStart, onTouchMove, and onTouchEnd props at

the appropriate times. We’ve also included an enabled prop to prevent dragging – we’ll use this

when the board is mounting and unmounting so that the user can’t interfere with the animation.

We chose to name our props onTouchStart, onTouchMove, and onTouchEnd after the touch

event names on the web, in order to make our Draggable component feel more familiar for

those with a web background. However, React Native doesn’t interpret these names in any

special way, and we could’ve named the props anything we wanted.

Using a function for children might look familiar. That’s because we used this same pattern

in the second part of the “Core APIs” chapter!

Let’s begin by writing the constructor. In the constructor, we’ll initialize this.state to keep track

of whether this component is actively being dragged. We’ll also create a new pan responder with

PanResponder.create.

When we write the render method, we’ll pass the value of this.state.dragging to the Draggable

component’s children. This allows them to render differently depending on whether a drag is

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occuring or not. In our case, we will use this to adjust the zIndex of puzzle pieces so that the piece

currently being dragged renders on top of the rest.

Add the following constructor to Draggable.js:

puzzle/components/Draggable.js

constructor(props) {

super(props);

this.state = {

dragging: false,

};

this.panResponder = PanResponder.create({

onStartShouldSetPanResponder: this.handleStartShouldSetPanResponder,

onPanResponderGrant: this.handlePanResponderGrant,

onPanResponderMove: this.handlePanResponderMove,

onPanResponderRelease: this.handlePanResponderEnd,

onPanResponderTerminate: this.handlePanResponderEnd,

});

}

We’ll implement each of the pan responder functions as instance properties of our component.

PanResponder functions

Let’s start by implementing this.handleStartShouldSetPanResponder, which we assigned to

onStartShouldSetPanResponder when creating our pan responder. We want this component to

become the responder when the enabled prop is true. Add the following function to the Draggable

component:

puzzle/components/Draggable.js

handleStartShouldSetPanResponder = () => {

const { enabled } = this.props;

return enabled;

};

Next, we’ll handle when this component becomes the responder in handlePanResponderGrant.

When this component becomes the responder, we want to set state.dragging to true. Add

handlePanResponderGrant now:

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puzzle/components/Draggable.js

handlePanResponderGrant = () => {

const { onTouchStart } = this.props;

this.setState({ dragging: true });

onTouchStart();

};

We’ll pass a the onTouchStart() prop later to allow the parent to animate the scale of the puzzle

piece when a drag begins.

Any time the user moves their finger, we’ll call the onTouchMove prop with the offset from the initial

touch position. This lets us animate the transform of the puzzle piece as it’s dragged from within

Board. Let’s do this by adding the following handlePanResponderMove:

puzzle/components/Draggable.js

handlePanResponderMove = (e, gestureState) => {

const { onTouchMove } = this.props;

// Keep track of how far we've moved in total (dx and dy)

const offset = {

top: gestureState.dy,

left: gestureState.dx,

};

onTouchMove(offset);

};

Lastly, when the user lifts their finger, or if the operating system cancels the gesture (e.g. a phone

call comes in), we want to reset our state and call onTouchMove and onTouchEnd with the final touch

position. We can handle both onPanResponderRelease and onPanResponderTerminate in the same

way. Add the following handlePanResponderEnd:

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puzzle/components/Draggable.js

handlePanResponderEnd = (e, gestureState) => {

const { onTouchMove, onTouchEnd } = this.props;

const offset = {

top: gestureState.dy,

left: gestureState.dx,

};

this.setState({

dragging: false,

});

onTouchMove(offset);

onTouchEnd(offset);

};

If the user receives a phone call, the drag gesture will end as if the user had lifted their

finger. This is acceptable for our game, but in some scenarios it may be desirable to handle

the onPanResponderTerminate case differently from an intentional touch event.

Rendering draggable pieces

When we render the Draggable component, we’ll call the children prop function. We’ll pass this

function the dragging and offset values from state so that we can update the positions of the

children as we render. We’ll also pass this.panResponder.panHandlers – these are the responder

functions wrapped by the pan handler to include gestureState.

Add the following render method:

puzzle/components/Draggable.js

render() {

const { children } = this.props;

const { dragging } = this.state;

// Update children with the state of the drag

return children({

handlers: this.panResponder.panHandlers,

dragging,

});

}

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Save Draggable.js. Now open Board.js again so we can render the Draggable component from our

renderSquare method.

There are three changes we’ll be making to renderSquare:

• We’ll return a Draggable component. We’ll move what we’re currently rendering into the

children function prop of Draggable. The children function is passed handlers, dragging, and

offset from the Draggable component, and should return the children component to render.

For the other props of Draggable, we need to set the enabled prop to true once the board’s

pieces have transitioned in – so we’ll check if the board is in the DidTransitionIn state. We’ll

also set onTouchStart, onTouchMove, and onTouchEnd props, which we’ll write shortly.

• We’ll update itemStyle to use the draggable argument of the children function. When

draggable is true, we want to set the zIndex of the piece to 1. This will ensure the piece renders

above the adjacent pieces when its being dragged (we’ll increase its scale during the drag, so

it will overlap the edges of the adjacent pieces).

• We’ll update the Animated.View by spreading the handlers argument of the children function

into the Animated.View props. This is how we apply the PanResponder to a view.

Let’s makes these updates to renderSquare now:

puzzle/components/Board.js

const itemSize = calculateItemSize(size);

return (

<Draggable

key={square}

enabled={transitionState === State.DidTransitionIn}

onTouchStart={() => this.handleTouchStart(square)}

onTouchMove={offset => this.handleTouchMove(square, index, offset)}

onTouchEnd={offset => this.handleTouchEnd(square, index, offset)}

{({ handlers, dragging }) => {

const itemStyle = {

position: 'absolute',

width: itemSize,

height: itemSize,

overflow: 'hidden',

transform: [

{ translateX: this.animatedValues[square].left },

{ translateY: this.animatedValues[square].top },

{ scale: this.animatedValues[square].scale },

zIndex: dragging ? 1 : 0,

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};

const imageStyle = {

position: 'absolute',

width: itemSize * size + (itemMargin * size - 1),

height: itemSize * size + (itemMargin * size - 1),

transform: [

{

translateX:

-Math.floor(square % size) * (itemSize + itemMargin),

{

translateY:

-Math.floor(square / size) * (itemSize + itemMargin),

};

return (

<Animated.View {...handlers} style={itemStyle}>

</Animated.View>

);

}}

</Draggable>

);

};

The above code snippet is pretty long, but we only actually made a few changes. One of the most

important lines is the one where we spread the handlers into the Animated.View:

puzzle/components/Board.js

<Animated.View {...handlers} style={itemStyle}>

Without this, our Draggable component won’t respond to touches!

For each of the Draggable props onTouchStart, onTouchMove, and onTouchEnd, we’ll call the Board

methods handleTouchStart, handleTouchMove, and handleTouchEnd, respectively. These handler

methods don’t exist yet, so let’s write them now.

Handling touch events

When a touch begins on a piece, we want to scale the piece so that it gives the illusion of lifting it.

We can do that by animating this.animatedValues[square].scale. We’ll use the Animated.spring

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function to scale the piece to 1.1 times its original size. Add the following handleTouchStart method

to the Board component:

puzzle/components/Board.js

handleTouchStart(square) {

Animated.spring(this.animatedValues[square].scale, {

toValue: 1.1,

friction: 20,

tension: 200,

useNativeDriver: true,

}).start();

}

As always, we have to call start() after setting up the animation.

Every time a touch moves, we want to update this.animatedValues[square].left and this.animatedValues[square].top

for the piece. We’ll need to restrict the piece’s movement according to which square is currently

empty. We can determine this by calling the availableMove utility function. This function returns a

string, 'up', 'down', 'left','right', or 'none', depending on the state of the game board. Based on

this, we can restrict the values of top and left to only allow the valid moves according to our game’s

rules. The exact math we use is specific to this game, so it’s not important to follow it completely.

Add the following handleTouchMove to Board:

puzzle/components/Board.js

handleTouchMove(square, index, { top, left }) {

const { puzzle, puzzle: { size } } = this.props;

const itemSize = calculateItemSize(size);

const move = availableMove(puzzle, square);

const { top: initialTop, left: initialLeft } = calculateItemPosition(

size,

index,

);

const distance = itemSize + itemMargin;

const clampedTop = clamp(

top,

move === 'up' ? -distance : 0,

Gestures 455

move === 'down' ? distance : 0,

);

const clampedLeft = clamp(

left,

move === 'left' ? -distance : 0,

move === 'right' ? distance : 0,

);

this.animatedValues[square].left.setValue(initialLeft + clampedLeft);

this.animatedValues[square].top.setValue(initialTop + clampedTop);

}

We have two options for updating the top and left Animated.Value. We could either

animate them (e.g. using Animated.spring), or update them immediately with setValue.

After testing both ways, we found using setValue provides a slightly better experience on

slower devices in this specific case.

Before we handle when touches end, let’s first write a utility method, updateSquarePosition, that

animates a piece’s position. When updating a piece’s position, we’ll update both the top and left

values at the same time for simplicity (even though a piece can only be moved along one axis at a

time).

Begin the updateSquarePosition method with the following:

puzzle/components/Board.js

updateSquarePosition(puzzle, square, index) {

const { size } = puzzle;

const { top, left } = calculateItemPosition(size, index);

const animations = [

Animated.spring(this.animatedValues[square].top, {

toValue: top,

friction: 20,

tension: 200,

useNativeDriver: true,

}),

Animated.spring(this.animatedValues[square].left, {

toValue: left,

friction: 20,

tension: 200,

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useNativeDriver: true,

}),

];

Since we’re going to animate multiple values at once, we’ve created an animations array containing

all of our animation objects. Notice that we didn’t call start() on these animations. We need to know

when both animations have completed, which is hard to determine if we start them independently.

Fortunately, the React Native provides the Animated.parallel for this case.

Animated.parallel takes an array of animations, and returns an object with a start(callback)

method, just like with other animations. The callback is called when every animation in the array

is completed. To make our updateSquarePosition slightly more convenient to use (with async/await

syntax), we’ll return a Promise that resolves when the callback is called.

Update the updateSquarePosition method to call Animated.parallel and return a Promise now:

puzzle/components/Board.js

updateSquarePosition(puzzle, square, index) {

const { size } = puzzle;

const { top, left } = calculateItemPosition(size, index);

const animations = [

Animated.spring(this.animatedValues[square].top, {

toValue: top,

friction: 20,

tension: 200,

useNativeDriver: true,

}),

Animated.spring(this.animatedValues[square].left, {

toValue: left,

friction: 20,

tension: 200,

useNativeDriver: true,

}),

];

return new Promise(resolve => Animated.parallel(animations).start(resolve));

}

Now that we’ve finished updateSquarePosition, we can write the handleTouchEnd method to finish

handling touch events.

When a touch ends, we need to do a few things:

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• We need to scale the piece back to its original size (a scale value of 1) using Animated.spring.

• We need to detect whether the user dragged the piece far enough to move it or not. If the piece

was moved more than halfway into the empty square, then we’ll consider this a move, and

we’ll inform the Game of the move by calling the onMoveSquare prop. If the piece was moved

less than halfway into the empty square, then we won’t consider this a move, and we’ll instead

animate the piece back to its original position.

Begin the handleTouchEnd with the following to reset the piece’s scale:

puzzle/components/Board.js

handleTouchEnd(square, index, { top, left }) {

const { puzzle, puzzle: { size }, onMoveSquare } = this.props;

const itemSize = calculateItemSize(size);

const move = availableMove(puzzle, square);

Animated.spring(this.animatedValues[square].scale, {

toValue: 1,

friction: 20,

tension: 200,

useNativeDriver: true,

}).start();

Based on the direction the piece was moved, we can determine if it was moved more than halfway

to its destination. We’ll finish the handleTouchEnd method by either calling onMoveSquare to inform

the Game of a successful move, or by calling updateSquarePosition to reset the piece’s position:

puzzle/components/Board.js

handleTouchEnd(square, index, { top, left }) {

const { puzzle, puzzle: { size }, onMoveSquare } = this.props;

const itemSize = calculateItemSize(size);

const move = availableMove(puzzle, square);

Animated.spring(this.animatedValues[square].scale, {

toValue: 1,

friction: 20,

tension: 200,

useNativeDriver: true,

}).start();

if (

Gestures 458

(move === 'up' && top < -itemSize / 2) ||

(move === 'down' && top > itemSize / 2) ||

(move === 'left' && left < -itemSize / 2) ||

(move === 'right' && left > itemSize / 2)

) {

onMoveSquare(square);

} else {

this.updateSquarePosition(puzzle, square, index);

}

After a successful move, the Game will update the puzzle object and pass it back into the Board

as a prop, along with the previousMove prop. We need to handle updates to these props in

componentWillReceiveProps.

Whenever the puzzle object updates, we’ll call updateSquarePosition to animate the piece that was

last moved into its new position. To do this, add the following componentWillReceiveProps to Board:

puzzle/2/2/components/Board.js

async componentWillReceiveProps(nextProps) {

const { previousMove, onTransitionOut, puzzle, teardown } = nextProps;

const didMovePiece = this.props.puzzle !== puzzle && previousMove !== null;

if (didMovePiece) {

await this.updateSquarePosition(

puzzle,

previousMove,

getIndex(puzzle, previousMove),

);

}

Great! We’ve finished the touch event handling.

Try it out!

Save Board.js. After the app reloads, press the start button to begin the game and try dragging

pieces around.

Gestures 459

You should see the pieces smoothly scale up and down as you touch and release them. You should

only be able to drag pieces into the adjacent empty square on the board. The pieces should snap either

to their new positions or back to their original positions, but should never get stuck in-between.

Wrapping up gesture handling

We just built a drag-and-drop gesture from scratch! Let’s review what we did:

• We used a PanResponder to interact with the gesture responder system. We implemented

the onStartShouldSetPanResponder handler to request the global interaction lock, and then

we implemented onPanResponderGrant, onPanResponderMove, onPanResponderRelease, and

onPanResponderTerminate to keep track of the state of the gesture.

• We created a generic Draggable component that provides us with a simpler interface for han-

dling the gesture data we care about when building drag-and-drop interactions: onTouchStart,

onTouchMove, and onTouchEnd.

• We handled touches by looking at the offset from the first touch event to the current one,

and deciding whether or not to move the puzzle piece. We animated the scale of the puzzle

piece so that it feels like we’re lifting the piece off of the board. We animated the position of

the puzzle piece using Animated.spring and Animated.parallel to snap it to a valid position

on the board.

Gestures 460

• If a new game state was passed in as a prop, we animated the pieces to reflect the latest state

of the game board.

Putting it all together, we were able to achieve an intuitive-feeling, cross-platform drag-and-drop

gesture that performs smoothly even on lower-end devices.

Here are a few recommendations for building gestures:

• Most gestures should use the PanResponder like we did in this example, rather than using

the underlying View responder props directly (e.g. onStartShouldSetPanResponder instead of

onStartShouldSetResponder). This is because most gestures will need to use the values in the

gestureState provided by the PanResponder, which are tricky to calculate on your own.

• It’s often helpful to separate gesture-related code into a separate component like we did with

our Dragging component. This ensures we keep the state of the gestures (e.g. offset) separate

from the state of the board. It can also help with performance: the Draggable component will

re-render each time its state updates, but this is significantly less costly than if we had to re-

render a more complex component like the Board.

• If possible, test your gestures on different devices as you build them. You may find that your

intuition about what should perform better isn’t actually the case, and it’s much easier to work

through performance issues one-at-a-time in isolation, rather than all at once when the gesture

is finished. Furthermore, if you run into a React Native bug or inconsistency across platforms

(there are several of these), it’s better to know sooner than later.

Speaking of performance, there’s one more performance improvement we made which we haven’t

covered yet.

Using PureComponent

You may have noticed that our Board component extends React.PureComponent rather than

React.Component. Components that extend React.Component will re-render any time their parent

re-renders or any time setState is called internally, even if the value of state or props is the same.

By contrast, components that extend React.PureComponent will only re-render when state or props

actually change. React checks for changes using a “shallow” equality test (testing the top-level keys

and values within the state and props objects using ===). If state and props are unchanged, then

the component does not re-render.

Preventing re-rendering with PureComponent is especially important for our Board component. Each

time the Game increments the elapsed time counter in the top right, it will re-render all of its children,

including Board. If the Board re-renders every second, this means it will likely re-render during a

drag gesture. Re-rendering results in a noticeable stutter during the gesture. In other words, we use

PureComponent to achieve a smooth drag gesture even as other parts of the UI update.

Gestures 461

If you want to see this for yourself, find this line at the top of Board.js:

export default class Board extends React.PureComponent {

Change it to:

export default class Board extends React.Component {

Then save Board.js. You’ll notice the stutter as you drag pieces around, especially on slower devices.

Make sure to change this back when you’re done!

If we had to re-render the Board during drag gestures for some reason, we might instead consider

breaking the component into smaller components, some of which can re-render more quickly or can

extend PureComponent.

Finishing the game

We’ve nearly finished building the puzzle game now. The next part we’ll write is the animation

where the pieces fly onto and off of the board as it mounts and unmounts:

Gestures 462

Animating all pieces

Let’s start by updating the constructor of the board. Open Board.js if you don’t already have it

open.

We want the puzzle pieces to render off-screen to begin with. This will let us animate them into

view. We can use the Dimensions API we learned about in the chapter “Core APIs, Part 1” to get the

height of the screen, and we can use this when setting the initial Animated.Value for the piece’s top.

Update the constructor to:

puzzle/components/Board.js

constructor(props) {

super(props);

const { puzzle: { size, board } } = props;

this.state = { transitionState: State.WillTransitionIn };

this.animatedValues = [];

const height = Dimensions.get('window').height;

Gestures 463

board.forEach((square, index) => {

const { top, left } = calculateItemPosition(size, index);

this.animatedValues[square] = {

scale: new Animated.Value(1),

top: new Animated.Value(top + height),

left: new Animated.Value(left),

};

});

}

With this, pieces will initially render exactly one screen-height below where we’ll move them.

Next, we need to animate every piece onto the board. We’ll write a helper method animateAllSquares(visible)

that does this for us. This method will animate pieces onto the board when we call it with visible

set to true, and it will animate pieces off of the board when we call it with visible set to false.

To perform the animation, we’ll animate the top of each piece by mapping the puzzle.board

to an array of Animated.timing animations (although remember that we’re eventually rendering

translateY instead of top, for performance). Just like with our updateSquarePosition method, we’ll

use Animated.parallel to run all the animations at once and let us easily await their completion.

We can create a playful staggered animation by setting the delay option of the animation.

Add the following animateAllSquares method:

puzzle/components/Board.js

animateAllSquares(visible) {

const { puzzle: { board, size } } = this.props;

const height = Dimensions.get('window').height;

const animations = board.map((square, index) => {

const { top } = calculateItemPosition(size, index);

return Animated.timing(this.animatedValues[square].top, {

toValue: visible ? top : top + height,

delay: 800 * (index / board.length),

duration: 400,

easing: visible ? Easing.out(Easing.ease) : Easing.in(Easing.ease),

useNativeDriver: true,

});

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return new Promise(resolve => Animated.parallel(animations).start(resolve));

}

Notice that depending on the visible argument, we either animate the piece to its position on the

board, top, or off the board, top + height.

Let’s now update our componentDidMount to call animateAllSquares:

puzzle/components/Board.js

async componentDidMount() {

await this.animateAllSquares(true);

const { onTransitionIn } = this.props;

this.setState({ transitionState: State.DidTransitionIn });

onTransitionIn();

}

With that, we’ve completed the animation where the pieces fly onto the board! The last thing we

need to hook up is the animation where the pieces fly off the board when the game finishes.

We can do this by updating our componentWillReceiveProps to watch for when the teardown prop is

set to true by the Game. When the teardown prop first becomes true, we’ll call this.animateAllSquares(false).

Once this completes, we’ll set the transitionState to DidTransitionOut, and we’ll call the

onTransitionOut prop to inform that Game that the Board animations are finished and it’s ready

to be unmounted.

Update componentWillReceiveProps to:

puzzle/components/Board.js

async componentWillReceiveProps(nextProps) {

const { previousMove, onTransitionOut, puzzle, teardown } = nextProps;

const didMovePiece = this.props.puzzle !== puzzle && previousMove !== null;

const shouldTeardown = !this.props.teardown && teardown;

if (didMovePiece) {

await this.updateSquarePosition(

puzzle,

previousMove,

getIndex(puzzle, previousMove),

);

}

Gestures 465

if (shouldTeardown) {

await this.animateAllSquares(false);

this.setState({ transitionState: State.DidTransitionOut });

onTransitionOut();

}

Try it out!

Save Board.js. After the app reloads, press the start button to begin the game. To test the animation

we just added, you can either complete the puzzle… or you can press quit. The pieces should fly

offscreen, then the screen should transition out, and finally you’ll be taken back to the start screen.

From the start screen, you can begin another game. The state of the game completely resets after

each game.

Gestures 466

We’re Done!

We’ve built a complete puzzle game with animations and drag-and-drop gestures. Our app performs

smoothly and our animations look great on both iOS and Android.

In this chapter, we learned how to use the PanResponder in conjunction with the Animated API.

Using these together, we can build nearly any common gesture you can find in a mobile app.

We covered several ways of ensuring good animation and gesture performance:

• Using transform instead of top and left to avoid costly layout calculations

• Using useNativeDriver to reduce the number of messages that must be passed between the

native and JavaScript threads

• Using PureComponent to prevent unnecessary component re-rendering

In the next chapter, we’ll cover how to publish your app on the App Store and Play Store.

Native Modules

What are native modules?

So far in this book we’ve written all of our apps purely in JavaScript. We’ve used the built-in React

Native components and APIs to interact with the underlying native iOS and Android platforms.

However, sometimes we want to use native functionality that isn’t provided out-of-the-box by React

Native. In these cases, we can write native components and APIs ourselves, and expose bindings to

use them from JavaScript. In React Native, these bindings are called a “bridge.”

Common use cases

Native modules are most commonly used for bridging existing native functionality into JavaScript.

Native modules are also occasionally used for performance.

Here are a few of the most common cases:

• Accessing native platform features that React Native doesn’t support out-of-the-box, e.g.

payments APIs

• Exposing components and functionality when adding React Native to an existing native app

• Using existing iOS and Android libraries, e.g. authentication libraries for a 3rd party service

• High-performance algorithms like image processing that are usually low-level and multi-

threaded

• Extremely high-performance views when running into performance issues with React Native

views (this is rare)

When to use native modules

Using native modules should be the exception, rather than the norm. It’s generally best to write

views, algorithms, and business logic in JavaScript when possible. The JavaScript we write is almost

completely cross-platform, and updating to new versions of React Native is usually low effort. Native

modules, on the other hand, must be written per platform, and can be time-consuming to update

since they depend on both the native platform APIs and React Native’s APIs. Additionally, we can’t

use the convenient Expo preview app once we start working with native code – we have to “eject”

our app (covered later in this chapter) and build it using Xcode and Android Studio.

If we’re integrating React Native into an existing app (this is known as a “hybrid” app), it’s likely

we’ll use native modules more frequently, since we’ll want to expose the existing components

Native Modules 468

and functionality of our app to React Native. In the short term, it’s often faster to bridge existing

components than to re-write them in React Native. However, in the long term, it can be better to

re-write them – by migrating components to React Native, we’ll only need to maintain a single

implementation, and our team will only need knowledge of a single language/platform.

Native modules on npm

When we decide we need a native module, we should first check if there’s an existing open source

implementation. We’ll likely find an npm package for common use cases such as taking photos,

playing videos, and displaying maps.

The GitHub organization react-native-community

maintains several of the most popular

native modules. These modules are very high quality and maintained by React Native core

contributors.

It’s very important to read the installation instructions, as setup for native modules can vary.

Most native modules on npm come with two sets of instructions, one for automatic setup using

react-native link, and one for manual setup.

react-native-link

Most of the time, installing a native module consists of 2 steps:

1. Install the npm package with: yarn add foo

2. Integrate the native code into your app by running react-native link

Remember, yarn and npm work interchangeably, but you should always stick to one or the

other. Because we’re using yarn in this book, if you see npm install foo in a package’s

installation instructions, make sure to run yarn install foo instead!

The command react-native link can often integrate native modules automatically. Library authors

can configure the various paths and settings used by this command to integrate their native code.

However, react-native link only handles the most common cases, so many native modules come

with custom setup instructions beyond this. Custom setup instructions usually involve manually

modifying iOS and Android native code.

https://github.com/react-native-community

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Manual setup

If you’re building a hybrid app, it’s likely your directory structure and code will differ somewhat

from the structure expected by react-native link. For this reason, native modules usually

include a set of instructions for manually integrating the native code into your app. This generally

involves modifying the Xcode and gradle build configurations to compile native libraries that were

downloaded by yarn into the node_modules directory.

The react-native link command supports some configuration options by adding an rnpm:

{ ... } object to your project’s package.json. However, documentation is currently non-

existent. If you choose to try this, the source code for reading configuration options is

currently here

Building a native module

In this chapter, we’ll build an app that displays a native pie chart:

https://github.com/facebook/react-native/blob/6942408a474af80560497193f73c9e60bf272263/local-cli/core/ios/index.js#L34

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There are a variety of graphing libraries available for React Native already, some of which are written

in JavaScript and some of which are native modules. In order to explore how native modules work,

however, we’ll write a native pie chart module from scratch.

The semantics of native iOS and Android code are outside the scope of this book, so we will primarily

copy and paste the native code we need. People without any experience writing native code are often

able to bridge simple native modules, so we recommend you attempt to follow along even if you

don’t have any experience with these platforms.

Building this app will consist of the following steps:

1. Create a new app using create-react-native-app

2. Eject the app we just created so that we can modify its native code

3. Write the pie chart component for both iOS and Android

4. Create a single PieChart.js that renders the native pie chart component from JavaScript

If you’re primarily testing on Android, feel free to skip the Xcode/iOS sections, or vice

versa. The project will work correctly on one platform regardless of any native code or

development tools for the other platform.

Native development is challenging!

There are a lot of things that can go wrong when developing a native app. Although the code

we’ll write in this chapter is relatively simple (as native apps go), it’s likely you’ll run into several

challenges along the way, especially if you’ve never done native development before.

The most challenging issues tend to be related to your development environment or build tools.

These can be tricky to debug, since they may be somewhat unique to the setup on your computer.

When you encounter an error with a development tool or building the app, the best place to start

is with a Google search. This will often reveal a Stack Overflow question or GitHub issue where

somebody else in the community had the exact same problem. If you don’t find anything useful, we

recommend opening a GitHub issue on the React Native github repo

. This is the most likely way

to have your issue resolved in a timely manner. If that still doesn’t work, you’re welcome to ask us

(the authors) for help (instructions on how to do so are in the introduction chapter), but be aware

that it’s unlikely we will be able to solve problems related to native app development.

It’s not all bad news though! The React Native community is extremely active, and new native

modules are added to npm frequently – writing custom native modules will become less and less

common as the ecosystem evolves. These complex challenges with native development are a big

reason for React Native’s success after all!

https://github.com/facebook/react-native

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Development environment

Before we can get started building the pie chart app, you’ll need to set up your development

environment. If you haven’t done native app development before, it’s likely you’ll need to download

some new software. Building for iOS will require a computer running macOS with Xcode installed.

Building for Android can be done on any computer with Android Studio installed. We recommend

you set up at least one of these tools before continuing with this chapter.

To set up your development environment, follow the instructions on the “Getting Started”

page of

the React Native docs site. First click the “Building Projects with Native Code” tab at the top of the

page, then select the “Development OS” and “Target OS” you plan to test with.

Follow the instructions for the “Development OS” and “Target OS” of your choice, up until the

section “Creating a new application.” We’ll create a new application in a slightly different way than

these docs demonstrate (although both will give the same result).

https://facebook.github.io/react-native/docs/getting-started.html

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Initializing the project

Just as we did in the previous chapters, let’s create a new app with the following command:

$ create-react-native-app pie-chart --scripts-version 1.14.0

Once this finishes, navigate into the pie-chart directory.

Ejecting

Throughout this book, we’ve been using Create React Native App (CRNA) to set up new React Native

projects. This tool dramatically speeds up the process of creating a new project and previewing it

on your mobile device. By using the Expo client app for previewing, CRNA eliminates the need to

compile any native code. The Expo client app already contains a compiled version of all of the React

Native framework code, so apps written purely in JavaScript can be previewed within the Expo

client app by executing a JavaScript code bundle downloaded over the network.

However, as soon as we want to add custom native code, we won’t be able to preview our app using

the Expo client app. We’ll need to build our native code using Xcode and/or Android Studio. CRNA

can facilitate converting a pure JavaScript app into an app with both JavaScript and native code

through a process called “ejecting”. Ejecting a project copies the necessary native build dependencies,

Native Modules 473

configuration files, and scripts into the app’s root directory. After ejecting, we’ll be able to build

custom native code in our app, but we’ll no longer be able to use the Expo client app for easy

previewing.

Since there’s no way to undo an eject, if you’re ejecting an existing project, make sure the project is

backed up in a version control system. In this case, our pie-chart project is brand new, so backing it

up is optional.

From the root of the pie-chart directory, run yarn run eject to eject the project now. Upon running

this command, you should see the following prompt:

1 We didn't find any uses of the Expo SDK in your project, so you should be fine to ej\

2 ect to

3 "Plain" React Native. (This check isn't very sophisticated, though.)

5 We strongly recommend that you read this document before you proceed:

6 https://github.com/react-community/create-react-native-app/blob/master/EJECTING.md

8 Ejecting is permanent! Please be careful with your selection.

10 ? How would you like to eject from create-react-native-app? (Use arrow keys)

11 � React Native: I'd like a regular React Native project.

12 ExpoKit: I'll create or log in with an Expo account to use React Native and the Ex\

13 po SDK.

14 Cancel: I'll continue with my current project structure.

There are two different ways we can eject a project: either with or without ExpoKit.

If we’re importing any of the Expo APIs from within our app (e.g. import { Constants, MapView,

LinearGradient } from 'expo';), then we may want to consider ejecting with ExpoKit (the second

option). This allows us to continue using the Expo APIs.

If we’re not importing any Expo APIs, then we don’t need ExpoKit. That’s the case here, so we

should choose the first option and eject to a “regular React Native project.” The yarn run eject

command attempts to identify whether or not we’re using any Expo APIs and warn us – here, the

first line of the prompt indicates that our app doesn’t use any.

If we decide we want ExpoKit at a later time, we can add it to our app then.

Select the first option, React Native: I'd like a regular React Native project.

The next prompt will ask how you want to name your app:

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1 We have a couple of questions to ask you about how you'd like to name your app:

2 ? What should your app appear as on a user's home screen?

Type “PieChart” and hit enter to proceed.

The last prompt will ask how to name the Xcode and Android Studio project files.

1 ? What should your Android Studio and Xcode projects be called? (piechart)

Type “PieChart” and hit enter to proceed.

The script should then proceed with the ejection. After a few minutes (although it can take 10 or

more), you should see the following message:

1 Ejected successfully!

2 Please consider letting us know why you ejected in this survey:

3 https://goo.gl/forms/iD6pl218r7fn9N0d2

This means the project was ejected successfully!

Since we’re just building an example project, don’t fill out the survey!

There’s another tool for starting a new React Native project called react-native init. This was

the original way to get started with React Native, before create-react-native-app was created.

Using create-react-native-app is convenient, since you don’t need to have a full development

environment set up for both iOS and Android native apps.

Creating a new app with create-react-native-app then running yarn run eject is more or less

equivalent to running react-native init. If you know in advance that you’ll need to eject your

app to use native modules, then you should consider running react-native init for simplicity. We

demonstrated the ejection flow since we’ve been using create-react-native-app throughout this

book.

The “Getting Started” page of the React Native docs demonstrates how to use react-native init.

To learn more, continue reading from the “Creating a new application” section we recommended

you stop at earlier.

Project structure

Let’s take a look at the files in our project directory now:

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├── App.js

├── App.test.js

├── README.md

├── android

├── app.json

├── index.js

├── ios

├── node_modules

├── package.json

└── yarn.lock

Most of the files are the same as usual, but the ios and android directories and the index.js file are

new.

The ios directory contains an Xcode project and the android directory contains an Android Studio

project. From this point on, we’ll need to build the project in either Xcode or Android Studio before

we’re able to preview it in a simulator or on a device.

The index.js file is the “entry point” of our app now – in other words, it’s the first JavaScript file

in our app that gets executed when our app launches. Let’s look at this file now.

Open up index.js. You should see a call to AppRegistry.registerComponent. This registers a “root

component” of our app that will be instantiated by native code. Apps can have multiple root

components, each with a unique name, which can be instantiated within native code.

For apps created with create-react-native-app, the App.js file is normally the entry point

and the App component is registered automatically so long as it has export default in front

of it.

In this case, our root component will be named “PieChart”. Due to a bug in create-react-native-app,

the registered component name may not have the correct letter casing. If AppRegistry.registerComponent

is called with “piechart” (in lower case), replace it with “PieChart” now:

pie-chart/index.js

1 import { AppRegistry } from "react-native";

2 import App from "./App";

3 AppRegistry.registerComponent("PieChart", () => App);

Make sure to change the case of “piechart” to “PieChart” or the app will launch with an error

later! The error will say that no component named “PieChart” was registered.

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How native modules work

There are 2 kinds of native modules:

• API modules

• UI component modules

API modules expose bindings for native methods to be called from JavaScript. When calling a native

method from JavaScript, any values passed are marshalled

on the JavaScript side and unmarshalled

on the native side. All APIs called from JavaScript are asynchronous, so we will need to use promises,

callbacks, or events if we want to handle a response from the native side.

UI component modules expose a new React component that we can render from our JavaScript code.

When we render this component in our JavaScript, the native thread will use a “View Manager” to

create a new native view. The View Manager handles the lifecycle of the native view, including:

instantiating the view, marshalling and unmarshalling the component’s props, updating the view

with its props, and reusing native views where possible (for performance).

Prop types

On both platforms, we’ll want our view to consume the same props. This will allow us to create a

single React component that works for both iOS and Android. Our pie chart component will use the

following props:

pie-chart/PieChart.js

12 static propTypes = {

13 data: PropTypes.arrayOf(

14 PropTypes.shape({

15 value: PropTypes.number,

16 color: ColorPropType

17 })

18 ).isRequired,

19 strokeWidth: PropTypes.number,

20 strokeColor: ColorPropType,

21 ...ViewPropTypes

22 };

The different segments of the pie chart, the data prop, are passed as an array of objects containing a

numeric value and a color string. The segments of the pie chart may optionally be rendered with a

colored stroke, configurable with a numeric strokeWidth and strokeColor string. We’ll also allow

a style prop, just like other built-in React Native components.

https://en.wikipedia.org/wiki/Marshalling_(computer_science)

Native Modules 477

We’ll now build a UI component native module for each platform. As we build the module, we’ll

make sure it supports the data, strokeWidth, and strokeColor props. The style prop will be handled

automatically for us.

Feel free to follow the instructions for just iOS or just Android, and come back to the other

platform another time.

iOS

The structure of the ios directory looks like this:

1 ios/

2 ├── PieChart

3 │ ├── AppDelegate.h

4 │ ├── AppDelegate.m

5 │ ├── Base.lproj

6 │ │ └── LaunchScreen.xib

7 │ ├── Images.xcassets

8 │ │ ├── AppIcon.appiconset

9 │ │ │ └── Contents.json

10 │ │ └── Contents.json

11 │ ├── Info.plist

12 │ └── main.m

13 ├── PieChart-tvOS

14 │ └── Info.plist

15 ├── PieChart-tvOSTests

16 │ └── Info.plist

17 ├── PieChart.xcodeproj

18 │ ├── project.pbxproj

19 │ └── xcshareddata

20 │ └── xcschemes

21 │ ├── PieChart-tvOS.xcscheme

22 │ └── PieChart.xcscheme

23 └── PieChartTests

24 ├── Info.plist

25 └── PieChartTests.m

We’ll be opening PieChart.xcodeproj in Xcode, and then adding a few new files to the PieChart

directory from within Xcode. We won’t be adding native tests or configuring our app for Apple TV,

so we can ignore the PieChart-tvOS, PieChart-tvOSTests, and PieChartTests directories.

Native Modules 478

Swift or Objective-C?

When working with iOS, we have two choices for which language we want to use: Swift or

Objective-C (abbreviated Obj-C). The React Native framework for iOS is written in Obj-C, so the

built-in native modules and most of the documentation uses Obj-C. However, Swift is significantly

easier to learn and use, so we’ll be using Swift. In general we recommend using Swift for new native

modules, unless you know that you need Obj-C for some reason. Regardless of which we choose,

we’ll still need to use Obj-C for one part of the process.

Exporting the native view

There are 4 things we need to do in native code to create a new UI component native module:

1. Define the view that we want to instantiate from JavaScript. This will be a subclass of UIView.

In our case, we’ll call this class PieChartView.

2. Create a bridging header to expose our Swift code to React Native (this is only necessary if

we’re using Swift). Xcode will help us create this automatically when we create our first Swift

file.

3. Define the View Manager that will handle the lifecycle of our component. This will be a

subclass of RTCViewManager. We’ll call our subclass PieChartManager.

4. Export our custom View Manager and our component’s props to JavaScript using macros. This

is done in Obj-C, regardless of whether we’re using Swift for our component or not.

Creating files

Let’s begin by creating all the Swift and Obj-C files we’ll need within Xcode. Open PieChart.xcodeproj

in Xcode now. You can do this by launching Xcode and then choosing File > Open... from the

menubar.

This project was written in Xcode 9.3 with Swift 4.1. The project will likely fail to build in older

versions of Xcode!

First we’ll create our native view class, PieChartView.swift. In the project navigator on the left,

right click the “PieChart” group (the one with the yellow folder icon) and choose “New File…”.

Native Modules 479

Select the “Swift File” option.

Click “Next” and then save the file as PieChartView (the file extension is added automatically) in

Native Modules 480

the ios/PieChart directory:

Click “Create.” Upon creating this file, Xcode will prompt us to create a “bridging header” – this

is necessary to expose our Swift code to React Native, as React Native is written in Obj-C. Click

“Create Bridging Header”:

Native Modules 481

Next, right click the “PieChart” group in the project navigator again and create the file PieChartManager.swift

following the same process. Xcode won’t prompt you to create a bridging header this time, since we

already have one.

Last, we’ll create an Obj-C file to expose our view to JavaScript. Once again, right click the “PieChart”

group and choose “New File…”. This time, select “Objective-C File”:

Native Modules 482

Save this file as “PieChart” in the ios/PieChart directory.

At this point, the file navigator should show these files:

Now that we’ve created the files we need, let’s fill them out one by one.

Note that despite how the Xcode file navigator looks, PieChart-Bridging-Header.h is

actually in the ios directory, not the ios/PieChart directory. The Xcode file navigator

doesn’t map directly to the file system. There’s no need to move this file though – it will

work correctly regardless of where it exists on the file system. In fact, moving files managed

by Xcode can be fairly tricky, so for our pie-chart app, we recommend against moving any

files if possible.

Native Modules 483

You may notice that new files created in Xcode include a copyright header automatically. The

If you’re working on a real project, you’ll likely want to change the default value to your name or

your organization’s name, rather than “Facebook”.

The following image demonstrates how to change the organization name used for the copyright

header:

Bridging header

We’ll be copying code from the sample code directory, pie-chart/ios, into the files we just created.

Copy the contents of PieChart-Bridging-Header.h from the sample code directory into your own

PieChart-Bridging-Header.h:

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pie-chart/ios/PieChart-Bridging-Header.h

1 //

2 // Use this file to import your target's public headers that you would like to expo\

3 se to Swift.

4 //

6 #import "React/RCTViewManager.h"

This exposes the RCTViewManager.h headers to our Swift code so that we can write a native view in

Swift.

PieChartView

We’ll be using a subclass of UIView that draws a pie chart based on the data, strokeWidth, and

strokeColor props.

Copy the contents of ios/PieChart/PieChartView.swift from the sample directory into your own

PieChartView.swift file. We’ll look at two important details of this code.

If you don’t fully understand the explanations of the native code, that’s fine! This is mainly

included for people who do have a little native iOS development experience.

Props must be member variables of the class, exposed to Obj-C using the @objc annotation, for

example:

@objc var strokeWidth: CGFloat = 0.0

When a prop updates, we need to re-draw the pie chart. We can do this by overriding the didSetProps

method of our view:

override func didSetProps(_ changedProps: [String]!) {

setNeedsDisplay()

}

PieChartManager

Now that we have our view class, we need to create a manager class to instantiate it. Copy the con-

tents of PieChartManager.swift from the sample directory into your own PieChartManager.swift

file.

This class allows React Native to instantiate our PieChartView class as needed when we render it

from JavaScript.

Native Modules 485

Export macros

Last, copy the contents of PieChart.m from the sample directory into your own PieChart.m file.

pie-chart/ios/PieChart/PieChart.m

1 //

2 // PieChartExport.m

3 // PieChart

4 //

5 // Created by Devin Abbott on 6/3/18.

7 //

9 #import "React/RCTViewManager.h"

11 @interface RCT_EXTERN_MODULE(PieChartManager, RCTViewManager)

13 RCT_EXPORT_VIEW_PROPERTY(data, NSArray)

14 RCT_EXPORT_VIEW_PROPERTY(strokeColor, UIColor)

15 RCT_EXPORT_VIEW_PROPERTY(strokeWidth, CGFloat)

17 @end

This file uses macros to expose the view manager class and the view’s props to React Native.

The RCT_EXTERN_MODULE macro exposes our PieChartManager class to React Native. We can now

consume a native module called PieChart (without the Manager suffix) from our JavaScript.

The RCT_EXPORT_VIEW_PROPERTY macro exposes the member variables on our PieChartView class

as props to React Native. We also provide the types of these props so they can be marshalled and

unmarshalled correctly.

You may see an error 'React/RCTViewManager.h' not found on the line with the #import

"React/RCTViewManager.h". Xcode should find this dependency during the build process, so

the error should disappear once you build the app in the next step.

Try building!

We won’t be able try our component until we render it from JavaScript, but we can at least confirm

that our Xcode project builds successfully.

In the top left of Xcode:

Native Modules 486

1. Choose a simulator to build the project on. It’s more difficult to build on a real device, so we

recommend using the simulator for this chapter unless you’re already set up for building to

your device.

2. Click the play button to start the build.

Xcode will open a new terminal and start the packager in the root directory of our app if you don’t

have a React Native packager process running on your computer. This is for convenience – you may

also quit the terminal that Xcode opened and launch a packager process of your own with yarn

start as usual.

If everything goes well, after a minute you should see a “Build Succeeded” popup from Xcode. The

simulator you chose before building should launch (this can take a minute or two) and you should

see the default create-react-native-app screen.

Native Modules 487

Wrapping up iOS

We’ve finished creating a native pie chart component for iOS! The next step will be to consume it

from JavaScript. We’ll do that in the JavaScript section of this chapter.

At this point, you can either build the Android version of the pie chart, or skip ahead to the JavaScript

section (recommended) and come back to Android later.

Android

The structure of the android directory (excluding some deeply nested files) looks like this:

1 android/

2 ├── PieChart.iml

3 ├── app

4 │ ├── BUCK

5 │ ├── app.iml

6 │ ├── build

7 │ ├── build.gradle

8 │ ├── proguard-rules.pro

9 │ └── src

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10 ├── build

11 │ └── generated

12 ├── build.gradle

13 ├── gradle

14 │ └── wrapper

15 ├── gradle.properties

16 ├── gradlew

17 ├── gradlew.bat

18 ├── keystores

19 │ ├── BUCK

20 │ └── debug.keystore.properties

21 ├── local.properties

22 └── settings.gradle

We’ll be opening the android directory in Android Studio. We’ll be adding a few new Java source files

to android/app/src/main/java/com/piechart/. We can ignore the rest of the files in this directory,

which are mainly configuration files.

Java or Kotlin

When working with Android, we have two choices for which language we want to use: Java or

Kotlin. The React Native framework for Android is written in Java, so the built-in native modules

and most of the documentation uses Java. While Kotlin is more modern and a popular choice for

new Android apps, the vast majority of Android apps today are still written in Java, so we’ll be using

Java for our pie chart example.

Exporting the native view

There are 4 things we need to do in native code to create a new UI component native module:

1. Define the view that we want to instantiate from JavaScript. This will be a subclass of

android.view.View. In our case, we’ll call this class PieChartView.

2. Define the View Manager that will handle the lifecycle of our component. This will be a

subclass of SimpleViewManager<PieChartView>. We’ll call our subclass PieChartManager.

3. Define a new “package” called PieChartPackage, a subclass of ReactPackage, that instantiates

our View Manager.

4. Register our PieChartPackage at app launch from MainApplication.java.

Exporting the native view

Let’s begin by creating all the Java files we’ll need within Android Studio. Launch Android Studio

now, and choose “Open an existing Android Studio project”.

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Select the android directory within the pie-chart directory we just created and press “OK.” It may

take Android Studio a minute or two to configure the project.

Next we’ll create a new Java file, PieChartView.java. If you’ve never used Android studio before,

the following diagram depicts the steps to take. First, click the “Project” tab on the left of the window.

Second, choose the “Android” option in the dropdown at the top of the file tree. Third, right click

the “piechart” directory. Lastly, choose “New > Java Class” from the context menu.

Native Modules 490

Within the dialog that appears, type

PieChartView

and click “OK.”

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Repeat this process of creating new files for two more files: PieChartManager.java and PieChartPackage.java.

Once you’ve finished, the file tree should look like this:

Now that we’ve created the files we need, let’s fill them out one by one.

Native Modules 492

PieChartView

We’ll be using a subclass of android.view.View that draws a pie chart based on the data,

strokeWidth, and strokeColor props. React Native doesn’t interact with view classes directly, so this

class could be written any way we want – the member variables of the view could have completely

different names and types from our props.

Copy the contents of PieChartView.java from the sample directory into your own PieChartView.java

file.

PieChartManager

Now that we have our view class, we need to create a manager class to instantiate it. Copy the

contents of PieChartManager.java from the sample directory into your own PieChartManager.java

file.

This class allows React Native to instantiate our PieChartView class as needed when we render

it from JavaScript. The REACT_CLASS string determines the name of the component within our

JavaScript:

public static final String REACT_CLASS = "PieChart";

In this case, our component will be available as PieChart.

Methods of the

PieChartManager

handle updating the

PieChartView

to reflect the latest props. A

method can be registered to handle a specific prop by name using the @ReactProp annotation:

@ReactProp(name = "strokeWidth", defaultFloat = 0f)

public void setStrokeWidth(PieChartView view, float strokeWidth) {

view.strokeWidth = strokeWidth;

view.invalidate();

}

These annotated methods handle converting common JavaScript types to Java types. For more detail

on annotating props, check out this guide in the docs

Our PieChartView implementation overrides the draw method in order to do custom drawing, so

anytime we change a prop, we also call invalidate() to trigger a re-draw.

https://facebook.github.io/react-native/docs/native-components-android#3-expose-view-property-setters-using-reactprop-or-

reactpropgroup-annotation

Native Modules 493

Exporting the package

Now that we have our view manager class, we can create a ReactPackage subclass that registers it

with React Native. Copy the contents of PieChartPackage.java from the sample directory into your

own PieChartPackage.java file.

This class can register multiple native modules at once, including both API modules and UI

component modules. We register our PieChartManager using the following:

@Override

public List<ViewManager> createViewManagers(ReactApplicationContext reactContext) {

return Arrays.<ViewManager>asList(

new PieChartManager()

);

}

Lastly, we’ll register our PieChartPackage class within MainApplication.java.

The getPackages method currently looks like this:

@Override

protected List<ReactPackage> getPackages() {

return Arrays.<ReactPackage>asList(

new MainReactPackage()

);

}

Update it to the following:

@Override

protected List<ReactPackage> getPackages() {

return Arrays.<ReactPackage>asList(

new MainReactPackage(), new PieChartPackage()

);

}

This registers our PieChartPackage with React Native.

Try building!

We won’t be able try our component until we render it from JavaScript, but we can at least confirm

that our Android Studio project builds successfully.

Native Modules 494

We can either build to a real device or an emulator. It’s often quicker and easier to build to a real

device if you have one (and a USB cable) handy. If not, now’s a good time to set up an emulator. In

either case, follow the section titled “Preparing the Android Device”

for information on how to set

up your real device or emulator. If the website shows the wrong operating system (e.g. macOS when

you’re running Windows), you may need to set the “Development OS” toggle at the top of this page

again. Follow the instructions under “Preparing the Android Device” until you reach “Running your

React Native application.”

At the top of Android Studio, click the play button:

Next, choose a device or emulator to build the project on and press “OK”:

https://facebook.github.io/react-native/docs/getting-started#preparing-the-android-device

Native Modules 495

This will build the app, install it on your device, and launch it automatically. Android Studio doesn’t

automatically launch the React Native packager, so once the app launches you should see a red error

screen explaining that it can’t load the script.

Navigate to the root directory of the pie-chart app we just created and start the packager with yarn

start as usual. Once this finishes, press the “RELOAD” button at the bottom of the red screen on

your device or emulator.

If everything goes well, you should see the default create-react-native-app screen.

Native Modules 496

JavaScript

Now that we have a native implementation of our pie chart component, we can use it from JavaScript.

Using the native view from JS

In your text editor, create a new file PieChart.js in the root of our project directory (at the same

level as App.js).

In this file, we’ll import the native pie chart component, and render it from a new React component.

It’s best to wrap a native component within another React component so that we have the ability to

modify component props before they’re passed to the native component, and so that we can control

which imperative APIs are available on the component class (although in this case we won’t use

any).

Let’s begin by importing the following at the top of PieChart.js:

Native Modules 497

pie-chart/PieChart.js

import React from "react";

import PropTypes from "prop-types";

import {

ColorPropType,

StyleSheet,

ViewPropTypes,

requireNativeComponent,

processColor

} from "react-native";

Next, we’ll define a React component called PieChart. This component will wrap the native

component which we’ll import shortly. In the PieChart component, we need to define the prop

types and default props in the exact same way that our native component expects. So, we need a

data, strokeWidth, and strokeColor prop type. As a reminder, the data prop should be an array of

objects, where each object contains a numeric value and a color string.

Even though we didn’t specify this explicitly in our native code, our native component also expects

every prop type supported by the built-in React Native View. This is the default behavior for native

components, which allows our native pie chart to work with layout and gestures just like any other

React Native core component.

Create the PieChart class and propTypes now:

pie-chart/PieChart.js

export default class PieChart extends React.Component {

static propTypes = {

data: PropTypes.arrayOf(

PropTypes.shape({

value: PropTypes.number,

color: ColorPropType

})

).isRequired,

strokeWidth: PropTypes.number,

strokeColor: ColorPropType,

...ViewPropTypes

};

Note that we use the spread syntax, ...ViewPropTypes, to add every prop type from View to our

PieChart component.

Next we’ll add defaultProps:

Native Modules 498

pie-chart/PieChart.js

static defaultProps = {

data: [],

strokeWidth: 0,

strokeColor: "transparent"

};

requireNativeComponent

Before we add the render method to our PieChart component, we first need to import our native pie

chart component. We can do this using the requireNativeComponent(viewName, componentInterface)

API. In this case, the viewName will be "PieChart" (since that’s how we exported it from na-

tive code), and the componentInterface will be the PieChart component we just made. The

requireNativeComponent API will use the propTypes of our PieChart (the componentInterface)

to ensure that props passed from JavaScript to native code have the correct names and types.

Let’s add the following line below our PieChart React component definition:

pie-chart/PieChart.js

const NativePieChart = requireNativeComponent("PieChart", PieChart);

We’ll be rendering NativePieChart from within the render method of our PieChart component.

Even though we’re declaring NativePieChart below the PieChart class, we’ll still be able to

use it from within the render method. This works the same way as declaring styles below

the component class.

One last thing to do before writing our render method: we want to set the background of our native

pie chart component to "transparent", since native components have a black background by default.

Let’s add a style that handles this for us at the bottom of the file:

pie-chart/PieChart.js

const styles = StyleSheet.create({

container: {

backgroundColor: "transparent"

}

});

Native Modules 499

Rendering

Now we can write the render method of our PieChart class. Within the render method, we’ll

render a NativePieChart component, propagating all of the props passed to PieChart into the

NativePieChart.

There are two props that we’ll modify before propagating them to the NativePieChart component:

• data - we need to call processColor on each color within the objects in the data array. The

processColor function converts colors into a format that our native code can understand. React

Native normally handles this conversion automatically (e.g. for our strokeColor prop), but

since our colors are nested within objects/arrays, we must do it ourselves for the data array.

In order to do this, we’ll map each object in the data array to a new object containing the

converted color.

• style - we want to apply our default styles.container style before any other styles in order

to clear the default background color.

Let’s add the render method now:

pie-chart/PieChart.js

render() {

const { style, data, ...rest } = this.props;

const processedData = data.map(item => ({

value: item.value,

color: processColor(item.color)

}));

return (

<NativePieChart

{...rest}

style={[styles.container, style]}

data={processedData}

);

}

Save PieChart.js. That wraps up our PieChart component! Now it’s time to render this component

from App.

Native Modules 500

App

Open App.js. In this component we’ll render the PieChart with an initial set of data, and we’ll

include a button that randomizes the data.

We’ll begin by updating the imports at the top of the file:

pie-chart/App.js

import React from "react";

import { AppRegistry, StyleSheet, Text, View, Button } from "react-native";

import PieChart from "./PieChart";

Next, let’s add a style called chart in the styles object at the bottom of this file. This style will

render our pie chart as a 300x300 square with a margin below it. Update the styles object now:

pie-chart/App.js

const styles = StyleSheet.create({

container: {

flex: 1,

backgroundColor: "#fff",

alignItems: "center",

justifyContent: "center"

chart: {

width: 300,

height: 300,

marginBottom: 20

}

});

Now let’s move on to the App component. We’ll add a component state object containing some

initial data to render in the pie chart:

Native Modules 501

pie-chart/App.js

state = {

data: [

{ value: 12, color: "#2196F3" },

{ value: 12, color: "#8BC34A" },

{ value: 8, color: "#f44336" },

{ value: 4, color: "#FF9800" }

]

};

We’ll also add a randomize function to this class which randomizes the pie chart’s data:

pie-chart/App.js

randomize = () => {

const { data } = this.state;

this.setState({

data: data.map(slice => ({

value: Math.random() + 0.1,

color: slice.color

}))

});

};

Finally, we can update the render method. Delete the current placeholder contents within the render

method. Then, render a PieChart component using the styles and data we just defined, along with

a Button below it that calls this.randomize when pressed.

pie-chart/App.js

render() {

const { data } = this.state;

return (

<PieChart

style={styles.chart}

strokeColor={"white"}

strokeWidth={4}

data={data}

Native Modules 502

</View>

);

}

Save App.js.

Try it out!

You’ll likely need to manually reload the app on your simulator or device, since live reload and hot

reload are disabled by default in ejected apps.

To reload:

• On a physical device, shake the device until the developer menu appears, then tap “Reload”.

• On an iOS simulator, press Cmd+R.

• On an Android emulator, press R twice.

Once the app has reloaded, you should see the pie chart on the screen!

Try tapping the “Press to randomize” button a few times to make sure the native component updates

correctly whenever props change.

Native Modules 503

Wrapping up

We just created a native module for both iOS and Android, and bridged it into React Native.

Although bridging a native module is more complex than creating a JavaScript API or UI component,

it can be necessary in order to leverage native functionality.

As the React Native community has evolved, many native modules have been published on npm.

Creating native modules manually like we did in this chapter is already significantly less common

than it was several years ago. In the future, more and more apps will likely be built without writing

any native code. However, knowing how to bridge a native module will always be an important skill

for a React Native developer, since the need might arise throughout the lifetime of a project.

In the next chapter, we’ll cover how to publish our apps to the App Store and Play Store.

Building and publishing

After spending some time completing a mobile application, the next natural step is to share your

work with the world! If we want any user to be able to access our application at any time, we need to

build and publish it to an app store. By doing so, we let users discover and download our application

to their device.

How to read this chapter

This chapter serves as a reference for when you need to deploy and distribute a React Native

application. Unlike other chapters, you won’t be following along with an example application. Feel

free to read it now to get an idea of the process and return later when you’re ready to publish an

application of your own.

Shipping an application to an app store is generally a two-step process:

1. Create a native build of our application. This involves generating an IPA (iOS App Store

Package) file for iOS and an APK (Android Package Kit) file for Android.

2. Publish the build to an app store. App Store Connect

and Google Play Console

are the two

platforms used to publish and distribute an iOS and Android application respectively.

We’ll start with Building. There are two different approaches to creating a build. We’ll weigh their

advantages and disadvantages.

After this section, we suggest you head directly to the section covering the operating system that

you plan on publishing with: iOS or Android.

Building

We explored how to add native components to a React Native application in the “Native Modules”

chapter. While doing so, we described how to set up the necessary integrated development

environments (IDEs) needed for writing native code. To develop for iOS, Xcode

is the required

IDE and can only be installed on a Mac computer. Android Studio

is the official IDE used for

Android development.

https://developer.apple.com/app-store-connect/

https://developer.android.com/distribute/console/

https://developer.apple.com/xcode/

https://developer.android.com/studio/

Building and publishing 505

In order to submit to an App Store, we first need to create a build of our application that we can

publish. If we were building mobile applications without React Native, we would use the same IDEs

that we write native code with to create builds of our application. With CRNA however, we can

create standalone builds in two different ways:

1. Using Expo

2. Ejecting and creating builds manually through Xcode or Android Studio

Both approaches will allow us to generate iOS and Android builds that we can deploy to app stores.

Each approach has its advantages and disadvantages, which we will cover in a little bit.

In the next section, we’ll explore how Expo allows us to create standalone builds using one of its

tools. After that, we’ll review the pros and cons for using this approach as well as the trade-offs for

building manually using the IDEs.

Building with Expo

In the first chapter, we mentioned that Expo provides a number of tools

to simplify the process of

building and publishing React Native applications without using Xcode or Android Studio. Although

we’ve used the Expo Client extensively throughout the book to run and demo our projects, we have

not explored any other features provided by the platform. Expo provides two local development

tools that allow us to preview, share and publish our projects:

1. XDE

, or the Expo Development Environment, is a desktop app that we can use for macOS,

Windows, or Linux.

2. exp

is a command line interface.

XDE does not allow developers to create standalone builds to deploy to App Stores. For that reason,

we will be using the exp CLI throughout this chapter. We can begin by installing it globally:

yarn global add exp

In order to use any of the commands or services provided by exp, you need to have an Expo account.

You can create one at https://expo.io/signup

100

. Once your account is set up, you can sign in directly

through the command line:

https://expo.io/tools

https://github.com/expo/xde

https://docs.expo.io/versions/latest/workflow/exp-cli

100

https://expo.io/signup

Building and publishing 506

exp login

The app.json file is automatically generated in the root directory when creating a new project with

CRNA. The file allows us to modify a number of build configurations for our application:

{

"expo": {

"name": "Weather",

"slug": "weather",

"sdkVersion": "27.0.0",

"icon": "./path/app-icon.png",

"version": "1.0.0",

"ios": {

"bundleIdentifier": "com.companyname.appname"

"android": {

"package": "com.companyname.appname"

}

The attributes shown here are all the basic required fields needed to generate a native build:

• name: The name of the application that shows on the device home screen for applications

installed through an App Store.

• slug: The URL name for published Expo applications. For this configuration, the URL will look

like expo.io/user_name/weather.

• sdkVersion: The Expo sdkVersion that the application is running with. This needs to match

with the installed version in package.json.

• icon: The icon of the application that shows on the device home screen.

• version: The version of your current build.

• ios/android: iOS and Android build-specific configurations. The iOS bundleIdentifier and

Android package fields are both unique strings that identify the application and follow a reverse

domain naming convention. It is important to make sure a proper name is chosen for both fields,

as they cannot be changed once the application is published to an App Store.

Building and publishing 507

Although these are the only fields that are required to create native iOS and Android builds,

there are many other configurations that we can add including defining a splash screen

101

modifying the app status bar

102

and using appropriate and platform specific app icons

103

Expo provides an extensive guide

104

for best practices when creating builds to publish to

App Stores. Further, Expo provides a full list of possible configurations

105

Pros and cons of building with Expo

Creating builds with Expo is only possible if we do not plan on including any custom native code

whatsoever. If we need to include any native dependencies in our application, we have to eject

from CRNA and create our builds manually using Xcode and Android Studio. If you have already

ejected, then skip right ahead to the “Building Manually” section of this chapter. Otherwise, we

highly recommend to use Expo as it is a much simpler and quicker build process.

The Expo platform supplies a CLI to automate the process of creating a native iOS or Android build

of our application. When building for both iOS and Android, signing files are required to identify the

author of the application. Expo can automatically generate these files for you. This method allows

us to create iOS builds without using Xcode and without owning a Mac computer. However, Xcode

is still required to publish the build to the iOS App Store.

Another significant advantage of using this approach is over-the-air (OTA) updates. By default,

applications published with Expo will always check for updates when launched. If a new version of

the app has been published with Expo, the updated build will be fetched and loaded automatically.

With this, we can release fixes and updates to our application without going through the process

of publishing a new version to the App Store. We’ll cover this feature, along with its limitations, in

more detail at the end of this chapter.

A drawback of this approach is the limited control it allows over the build process. The final bundled

build will include every API provided by Expo, regardless of whether we use them or not. This can

result in significantly large build sizes even if the code that makes up our application is relatively

small.

Pros and cons of building manually

We can create builds of our application using the IDE for each platform, Xcode and Android Studio.

The primary advantage of this approach is the capability to add native iOS and Android code to our

application. Building manually also allows us to take full control over the build process, and this

includes modifying any signing steps and adjusting the final build size ourselves.

101

https://docs.expo.io/versions/latest/guides/splash-screens.html

102

https://docs.expo.io/versions/latest/guides/configuring-statusbar.html

103

https://docs.expo.io/versions/latest/guides/app-icons.html

104

https://docs.expo.io/versions/latest/guides/app-stores.html

105

https://docs.expo.io/versions/latest/guides/configuration.html

Building and publishing 508

The downside is that in order to use an IDE to build our app, we have to eject from Expo. This means

that we will have to own a Mac computer to create an iOS build. And as mentioned in the previous

section, the process of creating builds manually is much more complicated than using Expo. We

recommend taking this approach only if you need to include native code for your application and

own a Mac computer (if you wish to build for iOS).

As we covered in the last chapter, we have two different options for ejecting. We can either

eject to a regular React Native project, or detach to ExpoKit. Ejecting to regular React Native

means we also lose access to APIs provided by Expo. Further, we no longer benefit from the

platform’s built-in OTA updates. To push new updates to our users without deploying a

new build version, we’d need to use a tool like Microsoft’s CodePush

106

. However, ExpoKit

allows us to build with the IDEs while retaining access to Expo APIs (and OTA updates).

Reference Guide

Now that we’ve outlined our options, the rest of this chapter will be a reference guide that you can

refer to when you need to build and publish a React Native application. You can skip directly to the

operating system that you are currently working with.

iOS

As we mentioned previously, you can skip directly to the appropriate build method depending on

the state of your application:

• If you have not ejected your application from CRNA and do not plan on adding any native

dependencies, you can read the next section and skip the section that explains how to use

Xcode to create a build.

• If you have an ejected application, skip the following section and head directly to “Creating an

iOS build with Xcode.”

After using one of the two strategies to create a build, you can head to the “Testing the final build”

section to learn how to test your final build before continuing to read how to publish to the App

Store.

Creating an iOS build with Expo

In order to create builds for iOS applications, you need to enroll in the Apple Developer Program

107

You can either enroll as an individual or an organization. As an individual, apps distributed in the

App Store are tied to your personal name. As an organization, apps are tied to a legal entity’s name.

106

https://github.com/Microsoft/react-native-code-push

107

https://developer.apple.com/programs/enroll/

Building and publishing 509

Once we have a developer account set up, we can build a native iOS bundle through Expo with the

following command:

1 exp build:ios

We are then asked how we would like to handle our credentials:

The credentials here refer to signing certificates and a provisioning profile needed to submit

applications to the App Store. We’ll go with the simpler option of letting Expo handle all of the

credentials here, but we will explore what each of these mean in detail in the next section.

Once we have selected the option for Expo to take care of managing our files, we’ll need to submit

the ID and password for our Apple Developer Account:

Although we specified we want Expo to handle all credentials, we still have the option to provide

overrides for specific certificates. Expo prompts us about these next. We want Expo to handle both:

Building and publishing 510

Next, the build process will begin and a URL is provided (e.g. https://expo.io/builds/unique-id).

This URL provides access to the current status of the build and you can follow along by reading its

logs.

Clearing Credentials

Expo will always check if a certificate and provisioning profile exist before asking whether

it should handle creating new files or allow you to provide them. If you wish to clear the

already available credentials, you can run exp build:ios --clear-credentials.

The build process can take a few minutes to complete. Once completed, another URL will be provided

that contains the generated .ipa build file. Pasting this link into your browser’s address bar will

begin downloading it to your machine.

Creating an iOS build with Xcode

As we mentioned earlier in this chapter, an account with the Apple Developer Program is necessary

in order to create a build ready for distribution. You’ll need to enroll

108

if you haven’t already.

Once enrolled, the first thing we’ll need to do is code sign our application. Code signing is the

process of using a digital signature to identify the application author’s identity. Signatures are used

to ensure that updates to an application are published by the same author.

With Xcode, we have two options for code signing an application:

1. Manually, where we provide all the resources ourselves

2. Automatically, where Xcode takes care of creating all the necessary signing credentials

Automatic code signing is recommended in the Xcode documentation as it simplifies the process

of creating all the required assets needed for signing. Xcode takes care of creating all the needed

credentials and we only have to provide our developer account without doing more additional work.

With this approach, Xcode will:

• Create the necessary signing certificates

108

https://developer.apple.com/programs/enroll/

Building and publishing 511

• Create an App ID

• Handle all the provisioning profiles needed

Manually code signing an application means that we will have to create all the needed assets in the

Apple Developer Console and assign them to our application ourselves. This approach gives us some

more control over which provisioning profiles and signing certificates we would like to use.

If you don’t need to use a particular profile or certificate for your application, you should use the

recommended approach of automatic code signing. We’ll cover this first. After reading, feel free to

skip the portion of this chapter where we cover how to manually code sign your application.

If you are interested in learning more about how provisioning profiles and signing certificates work,

or you would like to manually handle these credentials yourself, then you can head directly to

“Manual Code Signing”.

Xcode version 9.1 is used in this chapter. There may be slight inconsistencies to the UI displayed

in the screenshots if you happen to be using a later version of the software. However, the general

procedure should remain the same.

Automatic Code Signing

To access a React Native application using Xcode, we can open the /ios/AppName.xcodeproj file in

our project. An Xcode project file (.xcodeproj) contains all the source files and resources needed

for building and managing a project with the platform.

Once the file is opened with Xcode, you should see your application loaded as the main target. This

is what the default dashboard looks like for an open application:

Building and publishing 512

The bundle identifier in the Identity section of the General tab is a required field and is a unique

string that identifies your application. Once a build of your application is uploaded for submission,

this field cannot be changed. The version and build number fields also need to be completed and

will be used by App Store Connect to identify different versions of your application.

In the Signing section of the screen, there is a checkbox for Automatically manage signing. After

checking this field, a “Team” drop-down is displayed asking for a development team that can be used

to associate all the created signing credentials. In order to see your team as one of the drop-down

options, you must add an Apple Developer Account to Xcode. You can add this account by opening

Xcode � Preferences in the main menu.

Once a development team is selected, you should see a signing certificate assigned to your developer

account. Xcode takes care of creating this certificate along with a provisioning profile and assigns it

to your team. With a certificate set up, you can now head directly to the “Creating a build archive”

section to create a build of your application.

Manual Code Signing

To begin the process of manually code signing, we’ll need to sign in to the Apple Developer Portal

109

https://developer.apple.com/account/

Building and publishing 513

The Apple Developer Portal provides links to resources, guides and documentation. There are two

important tools provided by the platform that we will be exploring:

• Certificates, Identifiers & Profiles is where we set up signing certificates and profiles for our

application to identify them.

• App Store Connect is a collection of tools that allow us to manage and submit application

builds to the App Store. We will cover how to use this to publish an application in the next

section.

Let’s navigate to Certificates, Identifiers & Profiles and begin by creating an appropriate

signing certificate. Once we are at the certificates

110

screen, clicking the + icon on the right hand of

the screen will allow you to create a new certificate. Make sure iOS, tvOS, watchOS is selected in

the dropdown in the left navbar.

110

https://developer.apple.com/account/ios/certificate/

Building and publishing 514

Signing certificates are digital signatures used to perform actions while ensuring the application

is from the same source and has not been altered with. There are two certificate types relevant to

building and distributing an iOS application:

• Development (iOS App Development): Used for the development of an iOS application and

limits the number of devices that the application be installed on

• Distribution (App Store and Ad Hoc): Used for the distribution of iOS applications to App

Stores or for Ad Hoc distribution

If you need to continue development of a React Native application after ejecting from CRNA and

would like to handle code signing manually, you will need to create an iOS App Development

certificate and register your device. Since this chapter focuses on distributing an application, we’ll

select App Store and Ad Hoc and click continue.

These are many more certificate types available for other use cases (such as enabling push

notifications or building and distributing a macOS app). You can see a full list of them in

the Xcode docs

111

https://help.apple.com/xcode/mac/current/#/dev80c6204ec

Building and publishing 515

The next screen provides instructions to create a Certificate Signing Request (CSR) file from a Mac

computer.

A CSR file is a formatted and encrypted file that contains information that identifies a particular

source and is used to issue a certificate. You can follow the instructions provided to save a CSR file

somewhere on your computer. Once that’s done, click continue to proceed and upload the file. Once

uploaded, you should be able to download the certificate by clicking Download. Double-clicking the

certificate file (.cer format) will save it in your Keychain.

Keychain Access

112

is a Mac application that can store sensitive account information such

as passwords and certificates.

Let’s move on to creating an app identifier. This is also known as the App ID and is a unique string

that identifies a single application (or multiple applications) connected to a development team. We

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https://support.apple.com/en-ca/guide/keychain-access/what-is-keychain-access-kyca1083/mac

Building and publishing 516

can do this by navigating to the iOS App IDs

113

screen by clicking Identifiers/App IDs in the side

menu. To register a new App ID, we need to fill out a few fields:

• App ID Description: Any description of your application can be used here, but this is usually

the same as the name of the app.

• App ID Prefix: This is your Team ID by default.

• App ID Suffix: By changing the suffix of our App ID, we can choose between its two different

types: * An Explicit App ID is used for a single application and must be unique. A reverse

domain naming convention is commonly used (com.companyname.appname). This option needs

to be selected in order to include most application services such as in-app purchases and push

notifications. You can see a list of all services that can be enabled/disabled at the bottom of the

screen. * A Wildcard App ID can allow a single identifier and provisioning profile to be used

for multiple apps. Many Apple services cannot be included with this type of ID. Since App IDs

cannot be changed for an application submitted to the App Store, it is important to ensure that

this option is only selected if no such services are included and will not be in the future. An

asterisk must be the last character of the ID (com.companyname.*) here.

• App Services: In here, you can select any Apple services to include in your application.

Once a certificate and identifier is created, a provisioning profile needs to be set up for the

application in order to allow it to be downloaded on physical devices. A provisioning profile is

the combination of an application’s unique bundle identifier and a signing certificate. Without a

profile, we cannot run an app on a mobile device.

A distribution provisioning profile is needed when an application is ready to be distributed to

multiple users. To create a distribution provisioning profile, we can click the Provisioning Profiles

–> Distribution

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list item in the side menu and then + on the right hand side of the screen.

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https://developer.apple.com/account/ios/identifier/bundle

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https://developer.apple.com/account/ios/profile/production/create

Building and publishing 517

Of the many possible options, there are two specific types of distribution profiles relevant to iOS

applications:

• App Store: Used to submit an application to the App Store.

• Ad Hoc: Used to distribute applications to multiple testers. Unlike a development provisioning

profile, an Ad Hoc profile can not used to debug applications.

Select App Store to create the profile you need to submit an application to the App Store. In the next

few screens, you will need to select the App ID you just created and the certificates you wish to

include in the profile. Once completed, you can name your provisioning profile and download it to

your machine.

Building and publishing 518

While creating a build of an application, a development provisioning profile uses a

development signing certificate to connect developers and devices to an authorized team.

This allows for multiple developers to debug on more than one device. The process of

selecting an App ID and certificate is the same, but each device’s unique device identifier

(UDID) needs to be explicitly added to the same profile. This can be done in the Apple

Developer Console

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The final step here is to manually assign our created distribution certificate to our project in Xcode.

We can open the /ios/AppName.xcodeproj file to do this.

The bundle identifier field must be the same as the AppID you created earlier in the developer console,

so you’ll need to make sure that it matches in order to create a successful build.

To manually take care of assigning a provisioning profile to our application, disable the Automatically

manage signing checkbox. Two newer sections, Signing (Debug) and Signing (Release), will

show up underneath. Select the Provisioning Profile dropdown for our signing release and click

Download Profile to download the distribution profile directly from the Apple Developer Console.

We can do the same for the debug section if we have a development profile created as well.

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Creating a build archive

Once all the credentials needed for code signing have been set up, we can create a build archive of

our application. We can begin by changing our device target at the top of the screen to Generic iOS

Device.

For an ejected CRNA application, we can run our application on different simulators using

the react-native run-ios --simulator="iPhone 8" command. We can also use this device

target menu on Xcode to do the same thing (as well as preview our application on a physical

device plugged in to our computer).

The Generic iOS Device target is only used to create an iOS device build. With this target selected,

we can select Product � Archive to create a build archive. After a few minutes, a window showing

all past archives will pop up. Clicking Export on the right hand side and selecting App Store as our

Building and publishing 520

method for distribution will allow us to export an .ipa file of our application. Instead of doing this

however, we can also submit directly to App Store Connect by clicking Upload to App Store. We’ll

cover the flow of this process in the next section.

Testing the final build

Since we signed the iOS build we created earlier in this chapter with an App Store distribution

certificate, it cannot be downloaded and run on a personal simulator/device for testing purposes

through Xcode. To allow for this, we can use TestFlight

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. TestFlight is an Apple platform that allows

others to download and test a production build on their devices. Instructions for using TestFlight to

invite users to test your production build can be found on the TestFlight website

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Publishing to the iOS App Store

Submitting and managing application builds for iOS can be done in App Store Connect

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. You can

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https://developer.apple.com/testflight/

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https://itunespartner.apple.com/en/apps/videos#testflight-beta-testing

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https://developer.apple.com/app-store-connect/

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App Store Connect provides a collection of tools that developers can use to manage their applications,

view user analytics, study trends and patterns as well as view financial results. Let’s navigate to My

Apps to view a dashboard of all the applications tied to your developer account. Nothing will show

here if you haven’t submitted an application before.

Click the + icon on the top left to add a new application. We’ll need to fill out a form with a few

fields:

• Platform: Select whether you’re creating a new iOS or tvOS application.

• Name: This is the display name of your application in the App Store.

• Primary Language: The main language your application is localized for.

• Bundle ID: The unique identifier for your application. If you let Xcode automatically code sign

your application or Expo handle creating credentials for you, you should see the ID for your

application in the dropdown. If you don’t see it, you may have to create your App ID manually

in the Developer Portal

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• SKU: This is the Stock Keeping Unit

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, an identifier commonly used for inventory and financial

tracking. It also needs to be unique to each application, and you can use the same string as your

bundle ID if you like.

Clicking Create will create our application on the platform.

We can add more general information here such as the application’s subtitle, privacy policy link, and

categories. On the left side of the screen, you’ll notice that the current status of our application’s

submission process is 1.0 Prepare for Submission. 1.0 refers to the version of the application, and

this number will change once we upload newer build versions. Clicking the link in the side menu will

navigate to another form that allows us to provide version-specific information of our application.

This includes:

• iPhone and iPad application previews and screenshots

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Building and publishing 523

• Promotional text

• Keywords

• App description

• App Store Icon

• App review information

All of this information can be modified at any time by submitting updated builds of an application.

In the Build section of the screen, you’ll see a message letting you know that you can submit builds

directly using Xcode.

Every iOS application that is published to the App Store goes through an extensive review process

before being accepted. To improve your chance of being accepted, there are a few resources you can

consider before submitting an application:

• Apple’s Review Guidelines

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• Xcode’s docs on App Store distribution

122

• Expo’s docs on app stores

123

In Xcode’s Window � Organizer screen, you can see a list of previously created build archives of

your application. This is what the screen looks like when you have multiple builds:

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https://developer.apple.com/app-store/review/guidelines

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https://help.apple.com/xcode/mac/current/#/dev91fe7130a

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Before we upload our build archive to the App Store, we can validate it to ensure that our build

files pass all the validation checks performed by App Store Connect. We can do this by clicking the

Validate button in the menu on the right. If any necessary configurations or assets are missing,

Xcode will give us an error here.

If our build archive is validated successfully, we can move on to clicking Upload to App Store. After

selecting the correct development team, we’ll see an Upload Successful message if there were no

issues with the archive.

It can take a few minutes for the build archive to show up in App Store Connect. Once it does, you

should see it in the Activity tab of the application. Moreover, the Build section of our submission

will now show a different message - Select a build before you submit your app. Clicking the

link will display a list of available builds.

After the build shows up on App Store Connect, it may still need to finish processing which

can take a little more time. At this point, you’ll see a (Processing) message next to your

build and you’ll only be able to select it for submission once it completes.

Once you select your build and complete all the necessary information for your application, you

Building and publishing 525

can submit it using the Submit for Review button. According to Apple’s support documentation

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review times vary. Half of all applications submitted are reviewed within 24 hours and over 90% are

reviewed within 48 hours. The status of the application in App Store Connect will be In Review

until it is completed.

If your application requires any form of authentication in order to be used, you should also

provide sign-in information for a dummy account in App Store Connect’s submission form.

Without this information, an application may not be able to be reviewed.

Android

As we mentioned previously, you can skip directly to the build approach that is more relevant for

your application:

• If you have not ejected your application from CRNA, you can read the next section and skip

the section that explains how to create a build manually

• If you have an ejected application, skip the following section entirely and head directly to

“Creating an Android build manually”

After using the appropriate strategy to create a build, we’ll discuss how to publish your application

in the “Publishing to the Play Store” section.

Creating an Android build with Expo

We can start a build process for Android with the following command:

1 exp build:android

We are then asked if we would like Expo to take care of creating a keystore or uploading it ourselves:

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A keystore is a file that contains private keys used to authenticate oneself. We’ll explore this in more

detail when we build an Android application manually later in this chapter, so we’ll go with the

option of letting Expo take care of it here.

Next, the build process begins and can take a few minutes to complete. As with the iOS build process,

a URL (https://expo.io/builds/unique-id) is provided that shows real-time logs from Android

Studio. Once the build process is finished, we’re provided a URL that contains the final .apk build

file. We can download the file by pasting the link into our browser’s address bar.

Clearing Credentials

A keystore file is used to represent the application owner’s identity, so keeping it secure is

extremely important. Moreover, we cannot submit updates to our application and publish

new versions if we lose our keystore file. We can clear a previously generated keystore used

by Expo with exp build:android --clear-credentials but it is important to make sure

we have fetched and stored a local version of the keystore file first. We can do that with exp

fetch:android:keystore.

Testing the final build after automatic signing

There are two different ways to test the final build of your application:

1. On an emulator.You can drag and drop the final build .apk to have it boot up automatically.

2. On a physical device. You will have to first install Android Platform Tools

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to your computer.

This includes Android Debug Bridge (adb), a command-line interface that allows you to control

an Android device connected to your machine. Once installed, you can run adb install

apk-file-name.apk with your device plugged in.

After setting up and testing your final application’s build file, you can head directly to the

“Publishing to the Play Store” section to learn how to publish your application as well as distribute

testing versions through different channels.

Creating an Android build manually

In order to code sign an Android application with a certificate, the build process requires a keystore

that contains an app signing key. This is done to ensure the source has not changed if any updates are

done to the application. There are two different ways to create an Android keystore for an ejected

React Native application:

1. Using Java Keytool

2. Using Android Studio

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Building and publishing 527

Using Java’s Keytool application can be quicker and is the approach mentioned in the React Native

documentation. For that reason, we’ll explain the process of using it here. However, instructions to

create a keystore using Android Studio can also be found in the Android Studio docs

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if you prefer

that approach.

Keytool

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is a command-line tool already included in the Java Development Kit that simplifies the

management of keys and certificates. It can be used to create a keystore containing an app signing

key used to sign an Android build.

The Java Development Kit (JDK) is a development environment to run and develop

applications built with Java. It is required to build Android applications with React Native

and you should already have it installed after ejecting and following the instructions

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build native Android code.

For Unix-like operating systems such as macOS and Linux, the keytool directory is added directly

to the $PATH variable of our system. This means we can run the command in any directory as an

executable. If you are using a Windows machine, you can only run keytool commands in C:\Program

Files\Java\jdk1.x.x_xxx\bin where 1.x.x_xxx is the version of JDK installed on your machine.

We can generate a keystore with a single key using the following command in our terminal:

$ keytool -genkey -v -keystore android-release.keystore -alias android-release-alias\

-keyalg RSA -keysize 2048 -validity 10000

The command contains a number of settings that would apply to our generated keystore:

• -keystore: The name of the keystore file. In here, the file would be named android-release.keystore.

• -alias: The keystore alias is its unique identifier and is used to code sign the application. Our

alias here is android-release-alias.

• -keyalg: Defines the algorithm used to create a public-private key pair in our keystore. RSA is

commonly used, but you can find out about all the possible options in more detail by referring

to Java’s cryptography architecture guide

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• -keysize: Specifies the size of the created key. In short, key size is used in cryptography to

define the number of bits in a key used in an algorithm. In here, we’ve specified 2048 bits

(which represents 256 bytes).

• -validity: The length of time our key will be valid in days. We’ve set our validity to 10000

days here.

These parameters are all we need to generate a working keystore for our application. You

can see a full list of possible configurations by typing keytool usage: to your terminal.

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https://docs.oracle.com/javase/8/docs/technotes/tools/unix/keytool.html

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Running the command will then prompt for passwords for the keystore and signing key, as well as

the Distinguished Name fields for your key. Once provided, an android-release.keystore file will

be created in the current directory.

Due to the fact that a keystore represents an application owner’s identity, it is extremely important to

remember to keep a keystore file secure. If the file is lost, updates to an application in the Play Store

cannot be performed and a new app will have to be created and submitted. If the file is compromised

or stolen, an attacker can publish a newer version of your application with malicious code. It’s

considered best practice to avoid committing a keystore to your version control system, like Git.

We need to place our keystore file in the android/app directory of our project. Once that is done, we

can configure the Gradle build process by adding our keystore and key information. Properties that

modify the build process can be added to the android/app/.gradle/gradle.properties file. Let’s

add the following to the bottom of the file:

APP_RELEASE_KEYSTORE_FILE=android-release.keystore

APP_RELEASE_KEYSTORE_PASSWORD=YOUR_KEYSTORE_PASSWORD_GOES_HERE

APP_RELEASE_KEY_ALIAS=android-release-alias

APP_RELEASE_KEY_PASSWORD=YOUR_SIGNING_KEY_PASSWORD_GOES_HERE

Gradle

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is an extensible and configurable build system used by Android to automate and

manage the entire build process. Instead of configuring Gradle to sign our application,

we also have the option of signing it manually

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using the apksigner

132

tool provided by

Android Studio.

Since these properties will need to be referenced in our build file, you can choose any key name for

each of these key:value pairs.

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https://gradle.org/

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In the above example, we added the username and password of both the signing key and

keystore in plain text to the Gradle properties file. This can be dangerous if you are working

on a project with multiple team members or if you submit your project to an open source

platform.

In these conditions, there are a few options that can be performed to keep this sensitive

information outside of this file:

1. Create a separate keystore.properties file to contain this information which can be

accessed in the application’s build file. Instructions to set this up can be found in the

Android Studio docs

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. The properties file should not be committed to version control

and can be added to the .gitignore file of the project so it is ignored by Git.

2. For macOS users, the credentials can be stored in Keychain Access and

loaded directly in the build file. Instructions can be found on this page:

https://pilloxa.gitlab.io/posts/safer-passwords-in-gradle/

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Once our signing credentials are added to our Gradle properties, we can access them in our build file

to complete the signing process. We can apply configurations to the build process by adding them

to the android/app/build.gradle file. Default configurations are added to this file as soon as we

create a new Android project.

Let’s add a signing configuration to the android block within this file underneath our default

configurations:

android {

// ...

defaultConfig {

// ...

}

signingConfigs {

release {

if (project.hasProperty('APP_RELEASE_KEYSTORE_FILE')) {

storeFile file(APP_RELEASE_KEYSTORE_FILE)

storePassword APP_RELEASE_KEYSTORE_PASSWORD

keyAlias APP_RELEASE_KEY_ALIAS

keyPassword APP_RELEASE_KEY_PASSWORD

}

buildTypes {

release {

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//. ..

signingConfig signingConfigs.release

}

We’ll now need to assign our newly created configurations as the release signingConfig of the build

process:

android {

// ...

defaultConfig {

// ...

}

signingConfigs {

// ...

}

buildTypes {

release {

// ...

signingConfig signingConfigs.release

}

And that completes our signing process! We can now navigate to the android/ directory and bundle

our application into a build:

cd android

./gradlew assembleRelease

Starting any Gradle build is done with the Gradle Wrapper

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, a command-line tool we can use at

the root of any Android project with /gradlew task-name. The assembleRelease command will

bundle (or assemble) all the core JavaScript code into the final build. This file can be found as a

app-release.apk file in the android/app/build/outputs/apk/ directory.

Instead of adding signing information to our Gradle properties, we can also manually sign

an application using Android Studio

136

or configure the build process

137

to automatically

sign any application. Both of these approaches will have the same end result.

135

https://docs.gradle.org/current/userguide/gradle_wrapper.html

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(https://developer.android.com/studio/publish/app-signing#release-mode)

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Testing the final build after manually signing

With an Android emulator running or a physical device connected to our machine, we can install a

production build of our application with the following command:

react-native run-android --variant=release

Aside from this, we can also publish alpha and beta versions of our app to a closed or open group

of testers before submitting it to the Play Store. This is covered in more detail in the next section.

Publishing to the Play Store

In order to publish and manage Android applications with Google Play Console

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, we’ll need to

create a Google Developer account

139

which requires a one-time payment.

Once we launch the console, we can click Create Application to begin the process of submitting

an application to the Play Store.

The first few fields we need to fill out are the application’s default language and title.

Once completed, we land on the

Store listing

page. This page allows us to specify the content in

our app’s listing in the Play Store. Parameters include:

• App description

• Graphic assets such as screenshots, icons and banners

• Application type and relevant category

• Privacy policy link

The left-side menu contains links to other areas of our application such as setting up the price of a

paid application, its places of distribution, and its content rating.

The Android doc’s Launch Checklist

140

is a great resource on a number of best practices.

The App Releases link in the menu is where we can manage uploading the APK builds of our

application. We can roll out new builds in a number of different channels:

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https://developer.android.com/distribute/console/

139

https://play.google.com/apps/publish/signup

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Building and publishing 532

• Internal test: Used to quickly publish builds for internal testing with a small number of testers.

• Closed (alpha): Used to publish builds for closed testing. This is useful to test your application

with a larger number of privately invited testers.

• Open (beta): Used to publish builds for open testing. Anybody can join this testing program

and provide private feedback to the author of the application.

• Production: Used to publish final production builds to the Google Play Store.

With any of these build tracks, we can upload an APK file by clicking Manage and then Create

Release. In here, we have the option of using App Signing by Google Play instead of managing our

signing keys manually. Enabling this feature will mean that the signing key within the keystore used

for your application will be handled as an upload key. If we opt-in to this feature, we will have to

publish a new application if our key is lost or compromised.

There are a few more noteworthy parameters we need to specify. The first is the release name. This

is not visible to users and is used to distinguish separate releases in the Play Store. It is recommended

to use the version of the APK here or an internal code name relevant to this release.

The second is release notes. We use release notes to explain modifications to the app in this release.

After this, we can upload the final signed .apk build file (app-release.apk in the android/app/build/outputs/apk/

directory) and review our submission before rolling it out! If you decided to roll out a production

build, the application should show up in the Play Store in a few hours.

Handling Updates

Expo supports over-the-air (OTA) updates by default. Every standalone application built with

Expo will know how to reference updates to an application based on its URL (expo.io/user_-

name/APP_NAME). Once an Expo-built application is published to the iOS App Store or Google Play

Store and installed on a user’s mobile device, we can distribute any updates to the application using

the following command:

exp publish

This command allows us to publish directly with Expo. When the app is re-launched on a user’s

device, the new version of the application will be automatically downloaded and displayed. With

this, we can publish newer updates without distributing new build versions to app stores.

Publishing with Expo can also allow Android users to test and demo a CRNA application

without having to go through the Play Store submission process. You can refer to the

Appendix to get a better idea of how this works.

OTA updates only work when JavaScript code is modified. We cannot use this feature if we eject

from Expo to a regular React Native project. In this scenario, we can use a similar third-party tool like

Building and publishing 533

Microsoft’s CodePush

141

. This will allow us to still benefit from publishing changes to our JavaScript

bundle without deploying to an app store. However, we need to be careful to not publish newer

JavaScript code over-the-air that bridges functionality to native modules.

In order to comply with Apple’s review process, it is important to make sure that only specific

fixes/modifications that are in-line with the general direction of the application are included in

OTA updates. Section 3.3.2 in the Apple Developer Program License Agreement

142

states that only

code that does not change the initial purpose of the application and does not bypass any iOS security

features are permitted.

Summary

We began the journey of learning React Native by building a weather app in the first chapter of this

book. With each subsequent chapter, we learned about important React fundamentals, explored all

of the core components and APIs provided by the framework, and covered advanced topics such

as animations and gesture controls. We spent an entire chapter learning how to apply different

navigation patterns to an application and even investigated how to build custom native modules and

bridge to them ourselves. If you’ve reached this point of the book, you have all the tools you need

to both build and publish a fully functional and powerful React Native application. Give yourself a

pat on the back!

As the authors, we hope you enjoyed reading this book as much as we enjoyed writing it!

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https://github.com/Microsoft/react-native-code-push

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https://developer.apple.com/services-account/download?path=/Documentation/License_Agreements__Apple_Developer_Program/

Apple_Developer_Program_License_Agreement_20180604.pdf

Appendix

JavaScript Versions

JavaScript is the language of the web. It runs on many different browsers, like Google Chrome,

Firefox, Safari, Microsoft Edge, and Internet Explorer. React Native takes this one step further and

allows us to write JavaScript to communicate with native iOS and Android components.

Its widespread adoption as the internet’s client-side scripting language led to the formation of a

standards body which manages its specification. The specification is called ECMAScript or ES.

The 5th edition of the specification is called ES5. You can think of ES5 as a version of the JavaScript

programming language. The 6th edition, ES2015, was finalized in 2015 and is a significant update.

It contains a whole host of new features for JavaScript. JavaScript written in ES2015 is tangibly

different than JavaScript written in ES5.

ES2016, a much smaller update that builds on ES2015, was ratified in June 2016.

ES2015 is sometimes referred to as ES6. ES2016, in turn, is often referred to as ES7.

ES2015

Arrow functions

There are three ways to write arrow function bodies. For the examples below, let’s say we have an

array of city objects:

const cities = [

{ name: 'Cairo', pop: 7764700 },

{ name: 'Lagos', pop: 8029200 },

];

If we write an arrow function that spans multiple lines, we must use braces to delimit the function

body like this:

Appendix 535

const formattedPopulations = cities.map((city) => {

const popMM = (city.pop / 1000000).toFixed(2);

return popMM + ' million';

});

console.log(formattedPopulations);

// -> [ "7.76 million", "8.03 million" ]

Note that we must also explicitly specify a return for the function.

However, if we write a function body that is only a single line (or single expression) we can use

parentheses to delimit it:

const formattedPopulations2 = cities.map((city) => (

(city.pop / 1000000).toFixed(2) + ' million'

));

Notably, we don’t use return as it’s implied.

Furthermore, if your function body is terse you can write it like so:

const pops = cities.map(city => city.pop);

console.log(pops);

// [ 7764700, 8029200 ]

The terseness of arrow functions is one of two reasons that we use them. Compare the one-liner

above to this:

const popsNoArrow = cities.map(function(city) { return city.pop });

Of greater benefit, though, is how arrow functions bind the this object.

The traditional JavaScript function declaration syntax (function () {}) will bind this in anonymous

functions to the global object. To illustrate the confusion this causes, consider the following example:

function printSong() {

console.log("Oops - The Global Object");

}

const jukebox = {

songs: [

{

title: "Wanna Be Startin' Somethin'",

artist: "Michael Jackson",

Appendix 536

{

title: "Superstar",

artist: "Madonna",

printSong: function (song) {

console.log(song.title + " - " + song.artist);

printSongs: function () {

// `this` bound to the object (OK)

this.songs.forEach(function(song) {

// `this` bound to global object (bad)

this.printSong(song);

});

}

jukebox.printSongs();

// > "Oops - The Global Object"

The method printSongs() iterates over this.songs with forEach(). In this context, this is bound

to the object (jukebox) as expected. However, the anonymous function passed to forEach() binds

its internal this to the global object. As such, this.printSong(song) calls the function declared at

the top of the example, not the method on jukebox.

JavaScript developers have traditionally used workarounds for this behavior, but arrow functions

solve the problem by capturing the this value of the enclosing context. Using an arrow function

for printSongs() has the expected result:

function printSong() {

console.log("Oops - The Global Object");

}

const jukebox = {

songs: [

{

title: "Wanna Be Startin' Somethin'",

artist: "Michael Jackson",

{

title: "Superstar",

artist: "Madonna",

Appendix 537

printSong: function (song) {

console.log(song.title + " - " + song.artist);

printSongs: function () {

this.songs.forEach((song) => {

// `this` bound to same `this` as `printSongs()` (`jukebox`)

this.printSong(song);

});

}

jukebox.printSongs();

// > "Wanna Be Startin' Somethin' - Michael Jackson"

// > "Superstar - Madonna"

For this reason, throughout the book we use arrow functions for all anonymous functions.

Classes

JavaScript is a prototype-based language where classes, which is common in many object-oriented

languages, were not used. However, ES2015 introduced a class declaration syntax. For example:

1 class Ball {

2 constructor(color) {

3 this.color = color;

4 }

6 details() {

7 return 'This ball is ' + this.color + '!';

8 }

9 }

11 class SoccerBall extends Ball {

12 kick() {

13 return 'This ' + this.color + 'soccer ball is kicked!';

14 }

15 }

This isn’t a brand new JavaScript model, but only a simpler way to define object oriented structures

instead of using prototypal-based inheritance. For context, let’s take a look at how this would

probably look like without using a class definition:

Appendix 538

1 function Ball(color) {

2 this.color = color;

3 }

5 Ball.prototype.details = function details() {

6 return 'This ball is ' + this.color + '!';

7 };

9 function SoccerBall(color) {

10 Ball.call(this, color);

11 }

13 SoccerBall.prototype = Object.create(Ball.prototype);

14 SoccerBall.prototype.constructor = Ball;

16 SoccerBall.prototype.kick = function () {

17 return 'This ' + this.color + 'soccer ball is kicked!';

18 }

We won’t be going into more detail explaining object oriented paradigms and structures in

JavaScript, but it’s important to realize that creating objects with properties can be simpler with

classes. The important thing to note here is that we use this exact same model to create our React

Native components.

If you’d like to learn more about ES6 classes, refer to the docs on MDN

143

Shorthand property names

In ES5, all objects were required to have explicit key and value declarations:

const getState = () => {};

const dispatch = () => {};

const explicit = {

getState: getState,

dispatch: dispatch,

};

In ES2015, you can use this terser syntax whenever the property name and variable name are the

same:

143

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes

Appendix 539

const getState = () => {};

const dispatch = () => {};

const implicit = {

getState,

dispatch,

};

Lots of open source libraries use this syntax, so it’s good to be familiar with it. But whether you use

it in your own code is a matter of stylistic preference.

Destructuring Assignments

For arrays

In ES5, extracting and assigning multiple elements from an array looked like this:

var fruits = [ 'apples', 'bananas', 'oranges' ];

var fruit1 = fruits[0];

var fruit2 = fruits[1];

In ES6, we can use the destructuring syntax to accomplish the same task like this:

const [ veg1, veg2 ] = [ 'asparagus', 'broccoli', 'onion' ];

console.log(veg1); // -> 'asparagus'

console.log(veg2); // -> 'broccoli'

The variables in the array on the left are “matched” and assigned to the corresponding elements in

the array on the right. Note that 'onion' is ignored and has no variable bound to it.

For objects

We can do something similar for extracting object properties into variables:

Appendix 540

const smoothie = {

fats: [ 'avocado', 'peanut butter', 'greek yogurt' ],

liquids: [ 'almond milk' ],

greens: [ 'spinach' ],

fruits: [ 'blueberry', 'banana' ],

};

const { liquids, fruits } = smoothie;

console.log(liquids); // -> [ 'almond milk' ]

console.log(fruits); // -> [ 'blueberry', 'banana' ]

Parameter context matching

We can use these same principles to bind arguments inside a function to properties of an object

supplied as an argument:

const containsSpinach = ({ greens }) => {

if (greens.find(g => g === 'spinach')) {

return true;

} else {

return false;

}

};

containsSpinach(smoothie); // -> true

We can also do this with functional React components.

ReactElement

React Native allows us to build applications with a fake representation of the native views rendered

in our mobile device. A ReactElement is a representation of a rendered element.

Consider this JavaScript syntax:

React.createElement(Text, { style: { color: 'red' }},

'Hello, friend! I am a basic React Native component.'

)

Which can be represented in JSX as:

Appendix 541

Hello, friend! I am a basic React Native component.

</Text>

The code readability is slightly improved in the latter example. This is exacerbated in a nested tree

structure:

React.createElement(View, {},

React.createElement(Text, { style: { color: 'red' }},

'Hello, friend! I am a basic React Native component.'

)

In JSX:

<View>

Hello, friend! I am a basic React Native component.

</Text>

</View>

Overall, JSX presents a light abstraction over the JavaScript version, yet the legibility benefits are

huge. Readability boosts our app’s longevity and makes it easier to onboard new developers.

Handling Events in React Native

Using bind statements within a render() method and property initializers aren’t the only ways to

handle events. We can also take care of binding our event handlers in a class constructor:

export default class SearchInput extends React.Component {

constructor() {

super();

this.handleChangeText = this.handleChangeText.bind(this);

}

handleChangeText(newLocation) {

// We need to do something with newLocation

}

render() {

Appendix 542

const { placeholder } = this.props;

return (

<TextInput

placeholder={placeholder}

placeholderTextColor="white"

style={styles.textInput}

clearButtonMode="always"

onChangeText={this.handleChangeText}

);

}

Instead of using a constructor to bind our method, we can also also leverage ES6 arrow syntax to

achieve the same effect:

export default class SearchInput extends React.Component {

handleChangeText(newLocation) {

// We need to do something with newLocation

}

render() {

const { placeholder } = this.props;

return (

<TextInput

placeholder={placeholder}

placeholderTextColor="white"

style={styles.textInput}

clearButtonMode="always"

onChangeText={text => this.handleChangeText(text)}

);

}

Notice how this simplifies our syntax where we don’t need to continously set up bind for each of

our event handlers. We’re specifically using ES6 arrow syntax to pass in the callback:

onChangeText={text => this.handleChangeText(text)}

Appendix 543

In most cases this is just fine, but it’s important to realize that this callback will instantiate every

time TextInput here is rendered. This will also be the case if we use bind statements within our

component JSX like we did previously. In most applications, this is unlikely to pose any noticeable

performance issues due to additional re-rendering. However, binding our member methods within

the constructor actually prevents this from happening.

This is where using property initializers can come in handy:

export default class SearchInput extends React.Component {

handleChangeText = newLocation => {

// We need to do something with newLocation

}

render() {

const { placeholder } = this.props;

return (

<TextInput

placeholder={placeholder}

placeholderTextColor="white"

style={styles.textInput}

clearButtonMode="always"

onChangeText={this.handleChangeText}

);

}

By using this pattern, we can remove some boilerplate within our constructor method as well as

handle events in a cleaner fashion without causing additional re-renders.

Higher-Order Components

A Higher-Order Component (or HOC, for short) sounds complex, but the idea is simple: we

want a way to add common functionality (e.g data fetching or drag-and-drop) to many different

components. To do this, we write a function that takes an existing component and wraps it in

an enhanced component. Instead of changing the code of original component, a higher-order

component allows us to change the functionality by controlling how and when we show the original

component.

In code, a HOC is conceptually straightforward as well. To create a HOC, we’ll create a function

that accepts a component to wrap:

Appendix 544

const Enhance = OriginalComponent => {

return props => <OriginalComponent {...props} />;

};

It looks like there is a lot going on in the Enhance function, but it’s pretty simple. The function accepts

an OriginalComponent argument and returns a stateless component function.

JSX spread syntax

We cover spread syntax, {...props}, in the “React Fundamentals” chapter, but we haven’t

mentioned we can also use it for component props. Instead of having to know all of the

key-value pairs in the props, the spread syntax takes each of the props and sets them as

key-value pairs automatically.

For instance, if we have a props object that has two keys:

const props = {msg: "Hello", recipient: "World"}

In spread-syntax, JSX will make the resulting examples equivalent:

The HOC can also return a class component:

const Enhance = OriginalComponent => {

return class extends React.Component {

render() {

return <OriginalComponent {...this.props} />;

}

};

Providing a class name is optional. It’s generally a good idea to provide a meaningful name

specific to the purpose of the HOC, but in this case we don’t know what the example HOC

does so we omitted the class name.

Notice that we can return whatever we want in our HOC as the render() value. To display the

original component, we just have to return it as a React component in JSX (as we do above). We

could instead modify the props of the original component before rendering it, or we could display a

completely different component based on the state of our enhanced component.

To apply our HOC to existing components, we can call it with the original component to get the

enhanced component.

Appendix 545

const EnhancedComponent = Enhance(OriginalComponent);

We can then use it anywhere we could use the original:

...

render() {

return <EnhancedComponent />

}

...

Publishing with Expo

Instead of ejecting a CRNA application and going through the process of building and publishing

standalone native builds manually, we also have the option to publish with Expo. This can be done

using the following command:

exp publish

Expo will take a few minutes compiling JavaScript bundles in production mode before deploying it to

the platform. Once completed, a URL will be provided that looks like https://exp.host/@fullstackio/contact-list.

You can send this link to any user who can open your application directly through the Expo Client

app.

One benefit of this approach is that users can access our application without us having to set

up standalone builds. This means we don’t have to take the steps to create and deploy native

iOS/Android builds to the app stores. The only requirement is that users have the Expo Client

application installed on their device. Every sample application in this book has been published with

Expo.

A disadvantage of relying on Expo Client for application distribution is that it only works for users

with Android devices. Users with iOS devices are significantly limited in accessing unauthorized

applications. They need to be logged in to the same Expo account that has published the project

in order to open and preview the application through the Client app. Only Android users have the

capability to open any Expo application published to the platform using their Client app. This means

that we only have the option of deploying a native build to the iOS App Store to allow other iOS

users to test and view our React Native application.

Changelog

This document highlights the changes for each version of Fullstack React Native.

Be sure to check there to ensure that you have the latest revision.

Revision 5 - 2018-07-26

Revision 5 - Adds Native Modules chapters to the book

Revision 4 - 2018-07-20

Revision 4 - Adds Publishing & Gestures chapters to the book

Revision 3 - 2018-05-10

Revision 3 - Adds Animation chapter to the book

Revision 2 - 2018-02-28

Adds Navigation chapter to the book

Revision 1 - 2017-12-06

Initial version of the book